the genetics of the budgerigar

Transcription

THE GENETICS OF
THE BUDGERIGAR
THE GENETICS OF
THE BUDGERIGAR
By
F. A. E. CREW
AND
ROWENA LAMY.
-
PRINTERS:
-
WATMOUGHS LIMITED, IDLE, BRADFORD, AND LONDON.
To
the many f~ciers who, by
their interest, generosity
and encouragement, have
made the wrjdng of this
book possible.
CONTENTS.
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
Author's Preface
Introduction
Green, Blue, Yellow, and White
Greywing
Linkage of Blue and Dark .
Cinnamon
The Bi-coloured or Halfsider
New Mutant Characters
Methods for the GenetIcal Analysis of a
New Mutant Character
In-breeding
Terminology .
Glossary
Index
vii
1
13
38
42
55
64
82
88
101
109
116
121
AUTHORS' PREFACE
E have decided that it is impossible
adequately to convey ideas concerning
genetic fact and theory by means of
letters to inquiring breeders who, having more
or less incorrect notions of what a geneticist is
and does, yet seek assistance in the solution of
their problems. We have concluded that someone
has got to explain to the breeders of budgerigars
f"
what genetics is.
So we have written this book.
The facts of organic inheritance in the budgerigar with which it deals were discovered, not by
ourselves, but by others, especially by Dr. Hans
Duncker and Dr. Hans Steiner who are well
known to all breeders of budgerigars in this
country. We ourselves are fully aware of the
important studies made by these investigators
on the biochemical differences which exist between
the various mutant forms of the budgerigar,
and also of the systems of formul~ used by them
to express these differences. But though we
make the fullest use of their writings, we choose
fo offer an interpretation of the results of experimental breeding somewhat different from theirs.
To us it seems highly desirable that the facts
of inheritance in the budgerigar should be
considered in the lig~t of modern genetic
theory, and that out of them should be woven
a hypothesis that shall be in tune with current
genetical teaching.
W
Vll
It is not doubted by anyone that "hereditary
factors" exist, and that they exert their action
and produce visible results by influencing
developmental processes. But since it is impossible
to tell how many such factors are related to any
one developmental process, we shall not assigp
particular functions to particular genes; we shall
not speak, for instance, of the "yellow fat factor"
as if the production of yellow fat depended
entirely on the gene in question. The production
of yellow fat, and, indeed, any other normal
developmental process, probably depends upon
the harmonious working in time of many genes;
it is therefore to be expected that a change
occurring in anyone of these complementary
genes might cause an interruption or an inhibition
of the processes leading to the formation of
yellow fat. Though the change would occur in
only a single gene, the result produced would
be the suppression of a characteristic which
normally was the expression of many genes.
It would thus be quite correct to call the mutated
gene a "yellow fat inhibitor" if we wished to
name it according to the biochemical effect
produced by it, but we cannot by implication
conclude that the same gene, when unmutated,
is the "yellow fat" gene, since we have no reason
to think that the production of yellow fat depends
on it alone. Its connection with yellow fat may
indeed be quite remote. The experience of the
past twenty years has shown that it is perilous
to make assumptions concerning the functions
of unmutated genes. It is safe always to refer
to them as "wildtype or normal allelomorphs."
We shall not, therefore, assign definite roles to
viii
unmutated genes, and there is no reason why we
should do so in the case of mutated genes either,
since this is not essential to genetical study.
But since genes must be talked and written about
as such, it is necessary that they should be named
according to some simple method appropriate
to their visible expression.
We shall refer to mutated genes as actual
genes producing definite effects, and not as
absences of biochemical processes, since this
latter custom is contrary to modern teaching and
is responsible for a very widespread confusion
of thought between hereditary factors (genes)
and biochemical effects. It does not follow that
if a biochemical process fails in expression, a
corresponding hereditary factor is necessarily
"absent"! The genetic formulre in use among
budgerigar breeders, which represent recessive
genes as mere absences of "normal" genes,
are in our opinion both inaccurate and misleading.
It is not necessary for the breeder or the
geneticist as such to concern themselves with
biochemical differences. The simple and obvious'
thing is to use for the mutant gene the same name
which is applied to the mutant form. A blue
budgerigar carries the gene for blue, or the blue
gene (b); a yellow budgerigar carries the gene
for yellow or the yellow gene (y); and so on.
Nothing is postulated as to the nature of genes
or their relation to biochemical factors; they are
considered simply as definite hereditary units
responsible for definite mutant forms.
ix
In wishing to dissociate the work of genetical
analysis in the budgerigar from biochemical
analysis, our intention is not to deprecate the
value of the latter, but to point out the field
proper to each kind of study. And since it is
obviously a knowledge of pure genetics which is
needed by the breeder, and not that of biochemistry, we have concentrated on the methods
and teachings based on the most recent discoveries in this field as well as in the field of the
closely allied subject of cytology as far as this
offe.ts useful knowledge to the breeder.
It is to be understood that we do not claim
that genetics has anything to offer that will assist
the exhibitor to win prizes: we do claim, however,
that the science, because of the understanding
and the power it gives, can add enormow;ly to
the joy and to the sense of adventure thdt are
associated with the creation of new varieties of
domesticated animals.
Mr. F. S. Elliott and Colonel A. H. Wall have
been kind enough to read our manuscript, and
to them we are greatly indebted for much and
most useful constructive criticism. We wish to
thank them, and at the same time to make it
quite clear that they are in no way responsible
for any of the shortcomings which may diminish
the value of this book to the breeder of
budgerig;ars.
F.A.LC.
R.L.
INSTITUTE OF ANIMAL GENETICS,
UNIVERSITY OF EDINBURGH.
1935·
x
I
INTRODUCTION
HE budgerigar (Melopsittacus undulatus), a
native of Australia, was first introduced to
science by Shaw and Nodder in 1805, and
first brought to Europe by Gould in 1840. The
.specimens described and figured by these authors
were similar in plumage colouration to the light
green variety of to-day. Records show that at
this time, though rarely found in the coastal
regions of Australia, the budgerigar was to be
encountered in immense flocks on the inland
plains. Doubtless many people caught these
wild birds and endeavoured to keep them in
confinement, but at this time nothing was known
of their habits or their needs, and so these attempts
at domestication failed. But in or about 1840
Gould's brother-in-law, Mr. Charles Coxon, did
succeed in rearing several, and the two specimens
which Gould brought to England were from
among these.
As was to be expected, their quaintness and
beauty attracted a great deal of attention and the
demand for them and for a knowledge of how to
keep them healthy and fully functioning in con- I
finement grew apace. Soon. every ship from
Australia brought its quota, and quicldy the
budgerigar invaded North-west Europe, and for
it a new era had dawned. It had ceased to be
merely the food of hawks and of the Australian
aborigine (budgerigar, it seems, means "good
T
2
THE GENETICS OF THE BUDGERIGAR
food" in their dialect) and was about to become
one of the most popular of pets with strong
fanciers' societies to protect and advance its
interests. As time passed, there developed a
budgerig?-r industry, and in Belgium, Holland,
France, Germany, the United States, Japan, as
well as in Great Britain, there are now a number
of breeding establishments which have assumed
the size and importance of industrial undertakings.
These are monuments, not to man's greed, but
to his urge to tame and domesticate, and to the
beauty, the quaintness, and the adaptability of
the budgerigar, a bird that has brought great
happiness into the lives of countless men and
one to which captivity has come to mean fncndship.
The fancier is one who not only continually
attempts to improve upon the already established,
but one who also looks for and cherishes the
unusual, the unexpected; indeed, he attempts to
bring the novel into existence. Among wild
populations we expect to find uniformity, and if
"sports" do appear we expect them to be exterminated by one or other of the selective
agencies that exist in nature. The budgerigar,
in its native Australia, is our light green, and only
one other colour variety, yellow, has so far been
recorded in a wild population (1886). Bur we
know that under conditions of domestication
novelties of form and colour sooner or later
appear, to become the raw material of the maker
of new breeds and varieties.
It is commonly thought that the conditions
incident to captivity and domestication are
INTRODUCTION
directly responsible for the appearance of those
variations from the wild type which, when they
first appear, are popularly termed "sports." But
it is probably truer to say that domestication is
merely responsible for their recognition and
perpetuation. It is not uncommon to find that
a "sport" is not so viable, being handicapped in
competition with its wild type fellows, being more
difficult to rear, perhaps more easily seen by such
as prey upon the species, by reason of its outstanding characterisation. Even under conditions
of domestication, it is not uncommon to find
that a "sport," when it first appears and for a.
few years afterwards, is much more difficult to
rear, is inclined towards infertility, has a slower
growth-rate, and is more prone to ill-health than
are the specimens of a long-established stock.
This being so, it is not unreasonable to assume
that in the wild a "sport," far more often than
not, has a poor chance of surviving to reproduce
its kind. Furthermore, even if it did, a new
colour would in all probability not become at all
common for a considerable period of time if
its mate did not happen to be like itself; for, as
will be seen, the mating yellow X wildtype green
gives offspring which are all wildtype greens.
But the cogency of this particular argument will
be better appreciated as the study of the inherit-,
For the
ance of plumage colour proceeds.
moment, it is enough to submit that in all
probability the occurrence of the various new
colour varieties has not been due to the good or
bad conditions of husbandry to which the
budgerigar has been exposed since it was first
4
reared in confinement, but to the fact that in all
sorts of conditions there is an ever present
tendency on the part of all living things occasionally to throw "sports." For the present it is
not possible in the case of the budgerigar
deliberately to call the novel into existence; the
fancier has to wait for its sudden and unexpected
appearance and to ensure that when it does appear
it shall be perpetuated.
The budgerigar now claims the attention of
the geneticist, not only because it is an excellent
material for studies in heredity, now that there
are a number of true-breeding varieties, but also
because all these are known to have occurred
within the last sixty-five years in a species WhlCh
has never been crossed with any other. The
evolution of the budgerigar, the creation of new
types out of an old, has proceeded and is still
proceeding under our very eyes. Darwin, it
will be remembered, supported his argument for
evolution through natural selection by calling
upon the evidence to be derived from the effects
of artificial selection (selection by man) upon
wild forms brought under domestication. Just
as man had created the breeds and the varieties
by deliberate selection of variations as these
appeared, so, he argued, had the selective
agencies in nature fashioned the species, the
present out of the pre-existing. But for his
examples Darwin was forced to refer to such
forms as the domesticated fowl and pigeon.
And in both cases there exists grave doubt that
the modern breeds and varieties are descended
from but one single form; for we know now that
INTRODUCTION
the rock pigeon (Columba livia) can produce
fertile offspring with the allied species C. leuconota,
and, further, that more than one of the four wild
species of Gallus probably has contributed to
the make-up of the modern fowl. This being
so, a certain strength is removed from Darwin's
argument.
But the budgerigar is free from this objection:
all the varieties we know have had their origin
in a single wildtype, as will also all the many
and varied forms that the future will disclose.
For this reason alone it becomes imperative that
the budgerigar breeder of to-day shall record
fully all the circumstances which attend the birth
of a new variety, for in so doing he will be
writing a chapter in the evolutionary history of
the species. In the case of almost every other
domesticated animal nothing that is accurate or
of value is known of the most important earlier
history of the breeds and varieties; all that remains
savours of anecdote and myth. Everyone of
them would seem to have been made by some
crossing of two already existing forms, yet the
story is so vague that no one could use it again
as a recipe; knowledge of great interest and often
of great importance has been lost or has given
place to untrustworthy tradition. But nowadays
things are different: there are breeders' societies"
there are journals, there are scientists in plenty'
eager to co-operate with the fancier, and fanciers
equally eager to collabQrate with the scientist.
The pride of creating a new variety must, of
course, be for the breeder, but when this has
been done and the reward has been claimed, the
B
6
full and detailed story of how it was done should
be placed on record, for the breeder of budgerigars must regard himself as a student of experimental evolution and permit his colleagues to
share the joy of his discovery.
The actual value of an animal or plant as a
material for the experimental study of heredity is
determined by several considerations. In the
first place it must exist in the form of a number,
the larger the better, of true-breeding varieties,
since the essential method of such investigation
is the crossing of representatives of two dissimilar
varieties. Secondly, it must be such as can be
kept under circumstances in which the experimenter has as complete a control of the environment (food, housing, temperature, humidity, e\c.)
as possible, so that the results that are obtained
cannot have been influenced by forces external
to the experimental material itself. Thirdly, it
must reproduce freely under such conditions and
produce as large a number of offspring as possible
in as short a time as possible since the student of
heredity depends for his interpretations on the
relative numerical proportions of different types
among the progeny. The larger the number
the more significant these proportions become.
Since large numbers are required, it follows that
the experimental material must be cheap to buy,
rear, feed, and house, or else that the study m'lst
be a co-operative one, very many investigators
each raising a small number and pooling their
records. Finally, since genetics as a science is
now a mighty superstructure built upon the
simple yet firm foundations of Mendelism, the
INTRODUCTION
7
ideal genetical material must possess a small
number of chromosomes, and these must differ
among themselves in size and shape, for it is
no longer sufficient only to consider hereditary
factors, those hypothetical units of organic
inheritance: the student must think in terms of
genes and of the chromosomes that carry them.
In all respects save one, the budgerigar is
highly satisfactory as genetic material. It exists
in a number of true-breeding varieties; it is kept
by thousands of intelligent and interested breeders
under satisfactory environmental conditions;
pooled records in great mass exist, and out of
their analysis there can be constructed an entirely
satisfactory interpretation of the mode of inheritance of all the varietal colours so far recorded.
New colours continue to make their appearance.
But, alas, the budgerigar has some fifty or more
chromosomes, more than man himself, and this l
as will be seen as the story unfolds, means' that.
our knowledge of budgerigar genetics will
necessarily remain imperfect and incomplete.
We shall be discussing in some detail the
mechanism that is responsible for the distribution
of the chromosomes during cell division, the
actual mechanism, that is, concerned with the
orderly transmission of the hereditary characters.
from generation to generation, and it is desirable,
perhaps, even at this stage, to make certain'
emphatic statements concerning it. Of course,
as the tale proceeds, these statements will be
elaborated. They will also be repeated from time
to time in order that the reader may not escape
from them and from their implications. For the
8
moment, it will be sufficient if they are considered
carefully, and some meaning, it matters not how
much, extracted from them.
Unless the likeness of offspring and parent is
due to supernatural causes, it follows that, since
the only material link between the generations
is the fertilised egg, there must be found in this
fertilised egg, resulting from the union of the
sperm from the father with the egg from the
mother, the factors which determine this likeness.
The genes are exceedingly minute in size, but
the chromosomes themselves can be seen with the
aid of a microscope that magnifies about 2,000
times.
To demonstrate them one takes the
thinnest possible small slice of any tissue, featb::r
follicle, muscle, skin, ovary, or testis, for exampl,\
that is still growing (and so preferably one takes
a sixty-hour old embryo straight from the shell)
and kills it instantly by immersing it into a
fixative solution, such as formalin, which kills
without destruction or distortion, and then
stains it by immersing it in various solutions of
dyestuffs. Since different structures take up the
different stains in varying degrees, when such a
preparation is examined under the microscope,
one can see that the tissue is composed of cells
and that each cell has a nucleus within which there
is a deeply-staining material called chromatin. In
many of the cells, owing to the fact that they
were in the process of dividing into two at the
time when they were killed, one can see that this
chromatin exists in the form of a number of
rods and dots, the chromosomes. The number of
these chromosomes is constant in any species,
INTRODUCTION
9
and is characteristic of that species. The total
number is a multiple of two, and the chromosomes, since they differ among themselves in
respect of size and shape, are seen to exist in the
form of pairs, the members of a pair being
homologous, i.e. similar in size and shape. In the
ripe egg and sperm .the number of the chromosomes is only half that found in the cells of the
body. This reduction in number is due to the
fact that only one member of each pair of
homologous chromosomes is present. Fertilisation, which essentially is the fusion of the nucleus
of the egg with that of the sperm, is attended by
the restitution of the chromosome number
characteristic of the species, and so, in the fertilised
egg, there are to be found the chromosomes in
pairs, and of each pair one member was received
by way of the sperm from the father, the other
by way of the egg from the mother.
The budgerigar has twenty-five pairs or more
of homologous chromosomes. It is impossible
to state the exact number for the reason that
there are so very many of them that have the
form of minute dots and it is exceedingly difficult
to count them with any degree of accuracy.
...
I»~~
... .•....
•
~i·;;·...·.~;1
"-,,,~
.,~~
Flg 1
The chromosomes of the budgengar X 4500
10
Each chromosome carries its complement of
genes, each in its own particular place, and so,
since there are two chromosomes of a kind,
there are two sets of genes, their total number
being fixed and constant. The genes are the
agents responsible for the development of the
individual, ,and they determine the characteristics
that the individual shall possess. But it is quite
impossible to say whether any particular gene is
responsible for any particular developmental
process or dictates the assumption of a particular
character. Probably every process is guided by
the harmonious interworking of a large number
of genes. If anyone of the genes concerned in
the inception and control of some phase lYf
development, for example, the formation of the
black pigment in the wing feathers and in the
eyes of the budgerigar, becomes altered in any
way, the whole process might be disturbed.
This might happen in one of several ways.
The process might begin later than it normally
does, so that in the end there would be less
pigment than is usually present in the adult bird;
or less pigment might actually be formed; or its
production may be completely inhibited by the
failure of some antecedent necessary process
which is beyond our present scrutiny.
When such a change takes place we are n'Jt
justified in saying, for example, that the bird
has lost "the melanin factor." There is probably
no such factor. All that we can say is that the
process of melanin formation has been disturbed
or inhibited by the occurrence of a mutation in a
single gene. We can, of course, follow the
INTRODUCTION
II
inheritance of the mutated gene thereafter; we
can recognise its presence by the particular
effect it produces, even though we do not know
how it produces that effect.
\Y/e have said that a gene which occupies a
given position on a chromosome is identical in
every respect with the gene· which occupies the
corresponding position on the partner chromosome. But when a gene mutates, it is clear that
it can no longer be identical with its fellow: the
change which it has undergone is permanent and
can be handed on from generation to generation.
By appropriate methods the breeder can bring
together in the same bird two genes of the new
kind, or he may combine one gene of the old
kind with one of the new, or he may restore the
original pair of old genes. What he cannot do,
however, is to collect more than two of these
genes in one individual, because, as has been
stated, the number of genes on the chromosomes
is fixed and constant.
Now, it may happen and is known to happen
quite often, that a given gene may mutate again
and again, each time producing a more or less
different effect, so that there will come into
existence several forms of this gene, capable of
occupying the same position on a given chromosome. These different forms are all capable of
transmission from generation to generation. The
breeder can select any two of them and bring
them together in one individual. Genes that
have this relation to one another; that is, that
have originated from the same original gene,
and occupy the same position on a chromosome
IZ
are called allelomorphs (other form) of each other.
This relation of allelomorphism is a very important one; it must especially be remembered
that though there may exist five, six, or more
allelomorphs of a gene, there can never be more
than two of these in a single individual.
We have said that members of a pair of chromosomes are identical in every respect: they are
h01JJ()logous. They have the same shape and size
and contain the same number of genes. There
is, however, one exceptional pair of chromosomes
in respect of which the sexes differ. In the cock
this pair, like all the others, is composed of two
identical chromosomes . called the X-chromosomes,
but in the heh there is only one X-chromosonw,
and this has a partner, the Y -chromosome, which
is different in every way from the X. It is much
smaller, it has a different shape, and, most
important of all, as far as we know it carries no
genes. While the cock, therefore, carries a
double set of X -borne genes, the hen can possess
only one set.
The facts concerning the mechanism by which
the characters (details of structure and of
behaviour) of a parent are caused to reappear
in subsequent generations are few and simple,
and the breeder who has grasped them can have
no difficulty in applying them and their impliG'tions to his own results, or in using them to
decide upon the best plethod of approaching new
problems. He will recognise- the exact value of
selection and of in-breeding, and will know how
to regard a new "sport" when it appears and how
to perpetuate it without loss of time.
II
A
GREEN, BLUE, YELLOW,
and WHITE
T the present time the following colour
varieties are firmly established and widely
spread:-
Greens: Light.
Dark.
Olive.
Blues: Sky.
Cobalt.
Mauve.
Yellows: Light.
Dark.
Olive.
Whltes with blue suffusion.
Whites with cobalt suffusion.
Whites with mauve suffusion.
In addition, various combinations of all these
with greywing and cinnamonwing also exist.
Important questions to be asked and answered
concerning this medley of beautiful plumage
colours are: How many genes are responsible
for the production of each colour? Are these
genes placed all on different pairs of chromosomes
or on one and the same pair, or some on one
pair and others on other pairs? The answers to
these questions might be put into a couple of
sentences, but they would be of little value to
the breeder without a knowledge of the methods
by which it is possible to discover' and to
demonstrate such facts.
We shall therefore
approach the genetical analysis of colour differences from the practical side, showing how the
knowledge of the relation of 'colour to gene and
gene to chromosome may be acquired through
experimental breeding alone.
13
14
GREEN
It is necessary, when discussing the genetical
nature of the light green, to point out that this
colour stands apart from all the rest in that it is
outside the scope of exact genetical analysis. It
is impossible by any known methods to discover
how many genes co-operate to produce it,
though we know that a change in anyone of
quite a number of genes is sufficient to alter the
colour of the bird. Indeed, it is because several
of these genes have in fact become altered by
mutation at different times that we have the
whole range of colours known in the budgerigar
to-day. Each of the various colours, therefore,
depends upon a single mutated gene or upon
combinations of two or more of these genes.
But the colour, light green, depends upon an
unknown number of unmutated genes, which,
because they have presumably existed in the
species in a well-balanced and comparatively
stable condition for an indefinite period of time,
we refer to as wildrype or "normal" genes. The
study of the inheritance of plumage colour in
the budgerigar is therefore the study of a definite
number of mutated genes in relation to each
other and to their unmutated wildtype or normal
allelomorphs.
In the Dark green budgerigar we have the
case of a mutation which, without changing the
colour entirely, modifies the shade or tone of
its appearance. The action of this modifying gene
js, moreover, of a dominant nature, and is therefore
-observable when the gene is present in single
GREEN, BLUE, YELLOW, AND WHITE
15
dose, as will be seen from the following breeding
results.
If a light green budgerigar is mated to a Dark
green, the offspring, as every breeder knows,
will be 50 per cent. Dark greens and 50 per cent.
light greens. This is explained by the fact that
the Dark green parent carries a pair of dissimilar
genes on one of its chromosome pairs, one
member of this pair of genes being responsible
for the Dark shade of colour and the other for
the light; but since "Dark" is dominant to
"light" the bird appears Dark. The light green
parent, on the other hand, possesses two "light"
genes, and is consequently of the wildtype or
normal light green colour.
How then are these genes distributed among
the offspring of the Dark and the light mating?
We have agreed that the whole contribution of
parents to offspring must necessarily be contained
in the germ cells or gametes, the mature egg and
sperm. Now, unlike all the other cells of the
body, these carry only one member of each pair
of chromosomes, and so, when the egg and sperm
unite in the process of fertilisation, the new
individual thus arising comes to possess the
original double or diploid number of chromosomes. If, therefore, anyone pair of chromosomes
in either of the parents consists of members
dissimilar in respect of the genes, they carry, it
follows that the germ cells oCthe bird will differ
among themselves with respect to that pair since
each germ cell can contain only one member of
the pair. This is the case, in fact, with the Dark
green bird of the parental generation (PI)' On one
16
of its chromosomes it carries the gene "Dark,"
and on the partner chromosome the normal
allelomorph of Dark which we have called for
convenience "light." Half of its germ cells,
therefore, carry the chromosome with the "Dark"
gene, and the other half carry the partner chromosome with the "light" gene. In the light green
bird, on the other hand, all the germ cells are
alike in carrying on that chromosome a "light"
gene. And since fertilisation is at random, it
happens that the sperm (or egg) of the light
parent unites as often with the egg (or sperm)
bearing the "Dark" gene as with that bearing the
"light" gene of the other parent.
Diagramatically this mating may be represen 'ed
as follows. The rods represent chromosomes _ 'ld
the small cross strokes genes. The letter D
represents the gene "Dark" and the
sign the
normal allelomorph of "Dark." (It is usual to
represent normal allelomorphs in this manner.)
The individuals with the mating of which an
experiment is started constitute the first parental
generation (PI); their offspring constitute the
first filial generation (FI) (Fig. 2).
"Dark," therefore, is a mutant gene which is
dominant to its wildtype allelomorph. All socalled Dark birds: Dark green, Dark yellow and
cobalt (which is equivalent to Dark blue), carry
one Dark gene and one "light" gene, but all
light birds (light green, light yellow, sky or
light blue) carry two "light" genes. It may be
noted here that "Dark" is the only mutant gene
known in the budgerigar which is dominant to
its wildtype allelomorph. At this stage it will be
+
17
convenient to give the genetic formula for Dark
green. A genetic formula, it must be noted,
presents only those genes which differ from the
wildtype light green, and we therefore represent
a Dark green bird simply as
Dark Green
~ indicating that die
Light Green
X
~+
P,
gametes of PI
iertillsa tion
lark greens
;0 per cent
FIg. 2.
light greens
50 per cent
only difference between it and the wildtype
(!)
is that it possesses, instead of two light genes,
one light and one Dark.
The next mating to be considered is that of
Dark green by Dark green. In this case the germ
cells of both parents are of two kinds, one
carrying Dark and one light (Fig. 3).
18
+
+
-+
+
o
...,
0
0
.,'"
:>
.!:I
0
8....
<!)
c..
'0
"'I
19
The offspring of two Dark greens, it will be seen,
are of three different kinds, only 50 per cent.
resembling the parents. Of the other 50 per cent.,
half are light green, since they have inherited
"light" genes from both parents, and the remainder, having inherited Dark genes from both
parents, are olive. The olive budgerigar, therefore, possesses two Dark genes. Its genetic
formula is written
~.
It is said to be hOlllO,?ygous
for Dark, while the Dark green is only heterozygous
for Dark.
Now, if olive is mated to olive it will readily
be understood that only olive offspring will
result, since the germ cells of an olive bird all
contain the gene, Dark. For the same reason
light green by light green gives only light green,
for the germ cells of light greens all contain
light genes. It is only when the germ cells of
one or both parents differ in respect of one or
more genes that the offspring show corresponding
differences. We may consider next the mating
of Dark green X olive. The germ cells of the
Dark green, as we have seen, are of two kinds,
whilst those of the olive are all alike. The
offspring will be of two kinds, Dark green and
olive, and they should occur in equal numbers
(Fig. 4).
BLUE
Now we can consider the mating of green
with blue. Blue plumage colour is the result of
a mutation occurring in a single gene, the effect
of which ~s to suppress the development of the
20
yellow ground colour in the bird. The gene
which produces this effect, unlike the Dark gene,
is recessive to its wildtype allelomorph, and
therefore it is necessary for a bird to possess two
"blue" genes in order to exhibit the blue plumage
character. If it possesses but one blue gene, the
other being the wildtype allelomorph of blue,
Dark Green
Ohve
x
~ 0 j0
/~
~D
I
~~D
~0 ~0
~+ ~ 0
50 per cent
olives
50 per cent
dark greens
PI
gamate;, of PI
ferhhsatlon
its plumage colour will be green. We may say,
therefore, that green is dominant to blue, or
that blue is recessive to green, and we employ
a small initial letter b, as the symbol of the
recessive character and the recessive gene "blue."
If we mate, then, a sky blue budgerigar to a
green, and raise a crop of youngsters, we note th~t
the plumage colour of these is, without exception,
green. Nevertheless, if we represent diagramatically the transmission by parent to offspring of
their respective colour genes, it will be obvious
that everyone of these green youngsters must
21
carry one gene for blue inherited from their
blue parent. The germ cells of the blue parent
are all alike with respect to this blue gene. The
green parent, however, carries in all of its
germ cells, instead of the gene for blue, the normal
unmutated gene as found in the wild type, the
gene we name the "wildtype or normal allelomorph of blue." The process of fertilisation
then brings together a germ cell which carries
the blue gene and one which carries the wildtype
allelomorph, and the result is an individual
carrying both; thus:blue
hght green
H+
light green
heterozygous
foc blue
X
Hb
I
I
~+
~b
gametes of P 1
~+
FIg. 5.
It must be noted, then, that th~se Fl (first filial
generation) birds, though very, if not exactly,
similar in appearance to their green parent,
have a different breeding capacity with respect
to the blue gene: they are not pure green birds
c
22
~....
....0
...,.,.,
.:
0
.;:l
Ul
-
oj
'"
:S
....
S
oj
LL.
.....,
....
bO
N
LL.
...,
.,::I .,
CIJ~
.0
_Q
..c
+
/
<..l
;at
>,P<
~~
...c
-+--
+
...c
~ --++
.,
:§
cO
(f)t;~
.: .... s:I
~ ~ 8
..
,:f.
600 ....
X
..a
...Q
..c
+
..c:
+
/-+-
\
:::; ~&
bONO
:.::: 810
...,.,.,
+
<nUl
~
-t
+
•
::I.:"",
0., s:I
bl>"'"
>, .... <..l
N bD ....
0 .... .,
s..c:
Po
_g~~
but "heterozygous blues"; that is to say, when
they are old enough to breed they will elaborate
germ cells of two kinds, half of them carrying
23
the gene for blue, and half the wildtype allelomorph of blue.
Now, we mate these FI heterozygous blues
with each other and so raise an F2 (second filial)
generation (See Fig. 6).
In the F2 (second filial) generation we obtain
both green and blue birds in the ratio 3 green
to I blue, but though the green birds of this
generation all look alike, we know that they are
not alike genetically: two thirds of their number
are heterozygous blues like their parents, and
only one in every three is a pure green. The
genetic formula for heterozygous blue should be
written
~
showing that the bird carries the
mutant gene for blue on only one chromosome
of a pair, the wildtype allelomorph on the other
chromosome being represented, as usual, by a
+ sign.
A pure blue bird is represented as
~~
It is desirable that the reader should accustom
himself to the depiction of breeding experiments
in this manner, according to which the green X
blue exercise becomes:hghtgreen
+
+
+
+
+
light green
+
b
X
sky blue
b
Fl
b
X
+
b
b
+
b
lIght green heterozygous
for blue
2
fl
b
b
sky blue
1
F2
24
There is no difficulty about the formation of
the FI since there is only one kind of sperm and
one kind of egg produced by the individuals of
the Pb but the formation of F2 out of FI is
perhaps somewhat confusing. Take the
above
the line of the left
this
right
+ first
t
t
+
of the FI pair; combine
with the
+ above the line of the
of the FI pair; now combine it with the
b below the line of the
t;
t;
t;
rig~t
below the line of the left
now take the b
combine it with the
+ above the line of the right
now combine
it with the b below the line of the second ~ .
The series is now complete, each kind of sperm
united with each kind of egg.
It is sometimes necessary to distinguish birds
which look alike, a homozygous light green and
a light green heterozygous for blue, for example.
This cannot be done by means of a scalpel or
microscope: the breeding pen is the only test.
jIomozygous +
mated to
b
+ light green
~as
· ht
bl 1IS
green + receSSIve ue b
gIVeS -heterozygous for
b
blue
whereas
light green +
mated to
b
heterozygous-recessIve blue- gives
for blue
b
b
+
b
b
b
green
heterozygous
for blue
blue
25
Here, then, is the crucial test. Homozygous
light green mated to blue gives 100 per cent.
heterozygous greens, whereas light green heterozygous for blue mated to blue gives equal
numbers of greens (heterozygous) and blues.
That is to say: if you wish to find out whether a
bird exhibiting a dominant character is homozygous or heterozygous, you mate it to a bird
with the allelomorphic recessive colour, and if,
among the offspring, you get any with the recessive colour, then you know that the parent with
the dominant is heterozygous. You will have
noted, of course, that any bird with the recessive
character must necessarily be homozygous for
that character. A blue bird cannot carry the
gene, otherwise, of course, it would not be blue.
So you can always know the genetic constitution
of a bird which exhibits only recessive characters,
and you can use such as the test for the actual
hereditary constitution of such as exhibit the
allelomorphic dominant characters.
+
YELLOW
Since the case of the inheritance of yellow is
exactly similar to that of blue, we shall merely
summarize the facts by saying that yellow
plumage colour is the result of mutation in a
single gene, that the yellow gene (y) is recessive
to its wild type allelomorph, and that consequently when (in PI) a green bird is mated to a
yellow, the (FI) offspring all exhibit green
plumage colour, though they are in reality not
pure greens but "heterozygous yellows." When
these (PI) heterozygous yellows are bred together
26
they will produce (in the F2 ) both green and
yellow offspring in the ratio 3 green:1 yellow.
bght green hght yellow
light green heterozygous for yellow
+
+
hght green
+
+
+
x
x
+
y
y
+
y
y
+
hght green heterozygous
for yellow
2
3
F,
y
Y
hght yellow
1
So we can say that yellow and its' wildtYF':'
allelomorph are another pair of allelomorphic
characters, and that yellow is recessive to wildtype. The yellow gene came into being as the
result of a mutation in the .gene we know ~S the
wildtype allele of yellow.
WHITE
Now we can ask a question of considerable
interest. Blue and yellow are each recessive to
its own allelomorphic wild type gene. What is the
relation of blue and yellow to each other. The
facts are that when homozygous blue is mated to
homozygous yellow, it matters nqt which is the
male and which the 'female, the Pi birds are all
wildtype (light green), and in the F2 there are in
every 16 birds, on the average, 9 wildtype, 3
blue, 3 yellow, and I white. This 9: 3 : 3 : I
ratio in the F2 requires for its explanation in
27
terms of the chromosome theory the use of two
pairs of homologous chromosomes.
We have
already used one pair fot the b gen~. We will
demand another pair for the yellow gene. The
mating thus becomes:A yellow bird that
not blue
y
mated
to
IS
-
+
-
y
+
the "yellow"
chromosome
pair
the "blue"
chromosome
pair
X
A
+
blue bird that
not yellow
b
+
b
the "yellow"
chromosome
pair
the "blue"
chromosome
parr
IS
Each of these birds has two pairs of chromosomes
that are concerned in this experiment: we neglect
all the rest. Each has what we shall call the blue
chromosome pair and the yellow chromosome
pair. On a blue chromosome there will be either
the gene for blue (b), or else its normal allele (
(Allele is merely a simplification of allelomorph.)
On the yellow chromosome there
be found
either the yellow gene (y) or else its normal
allele (+). I!lto each gamete produced by the
yellow bird there will pass one member of each
chromosome pair; that is to say, there will be
one chromosome carrying the normal allele of
blue (+), and another carrying the yellow gene
(y). Into each gamete elaborated by the blue
bird there will pass one chromosome carrying
the blue gene (b), and 'another carrying the
normal allele of yellow (+). / The union of egg
and sperm will result in the reconstruction of
these chromosome pairs, and the genetic constitution of the FI individuals will be:-
+).
will
28
THE GENETrCs OF THE BUDGERIGAR
2
the 'yellow"
chromosome
pair
the "blue"
chromosome
pair
Fig. 7.
Now, when such an PI wildtype individual,
which manifestly is a light green heterozygous
for blue and also heterozygous for yellow,
proceeds to elabotate its gametes, owing to the
facts that each of these can only include one
member of each pair of chromosomes, and that
the two members of each pair differ one from the
other in respect of the genes they carry, four
kinds of gametes will be produced.
1
1
2
2
2
2
FIg 8.
I-Yellow chromosome I can
chromosome I
2 -Yellow chromosome I can
chromosome 2
3 -Yellow chromosome 2 can
chromosome I
4 -Yellow chromosome 2 can
chromosome 2
or so:the "yellow"
chromosome
2
fmd itself m the company of blue
fmd Itself
tn
the company of blue
fmd Itself m the company of blue
fmd Itself m the company of blue
the "blue"
chromosome
+
+
+
b
It 15 suggested that the reader should, from thIS pomt, colour the chromosomes,
usmg yellow for the yellow pair, blue for the blue paIr and brown for the
chromosomes
+
3.
y
+
4.
y
b
29'
And there will be two of such series of gametes~
one provided by the FI male, the other by the
FI female. The easiest way of illustrating the
results of fertilisation of the four kinds of eggs
by the four kinds of sperm is to use the checkerboard method (Fig. 9). You will note that again
it is assumed that the four kinds of eggs occur
in equal numbers, and that fertilisation is at
random; that is to say that it is purely a matter of
chance which kind of egg is fertilised by which
kind of sperm.
The four varieties of spermatozoa are placed
along the top of the large square; the four
varieties of ova along the left side of the square,
in each case one being opposite a column or row
of smaller squares.
Next, in each of the 16 small squares insert
2 pairs of chromosomes, a yellow pair and a
blue pair.
Now fill in the gene signs. Take the ovum at
the top of the left side of the large square; its
Put these on the top two
genes are
chromosomes, one on each, in small squares
I, 2, 3, 4. The genes on the second ovum are
+ and b. Puo these on the top two chromosomes in squares 5, 6, 7, 8. The genes on the
third ovum from the top are y and +. Put
these on the top two chromosomes in squares
9, 10, II, 12. The genes on the fourth ovum are
+ +.
30
THE GENETICS OF THE BUDGERIG,\R
~l + I~ I ~ ~ I~ I ~ +I~ I ~ ~I~I$
0
~
~I .., +1~lj
+1
+
aJ
1>I~ +I~ >1>1 ~
~ 1+ I ~ +I+I~ ~ 1+1 ~
+1+ll
OIl
~I +1~lj +I>-I~
......
....
t-
M
<::>
....
"".....
+I~ >I>I~
l()
......
~I +1~ 1~ ~I~I~ +1~ 1~ ~1~1~
OIl
~1+lj
0
Q
""
......
..,......
+I~
+I+I~ ~I+I~ +I+I~ ~ 1+ I ~
+1+1
+I~
+1+lj
+1
~
bD
bD
bD
~1+lj >-1+lj
+1+lj +1+lj
.....
a>
l()
bD
M
......
+1
~I
+1
~I
+1
+1
~I
>-1
Ova.
3I
y and b. Put these on the top chromosomes in
squares 13, 14, 15, 16.
Now the sperm. The sperm on the left above
square I has
and
Put these on the bottom
two chromosomes in squares I, 5, 9, 13. The
second sperm above square 2 has
and b.
Put these on the bottom two chromosomes in
squares 2, 6, 10, 14. Sperm 3 has y and
These go on to the bottom chromosomes, one
on each, in squares 3,7, I I , 15. The fourth
variety of sperm, that above square 4, has genes
y and b. These go on the bottom chromosome
)n squares 4, 8, 12, 16; and the task is finished.
Fertilisation thus typically yields sixteen individuals in the F2 , and of these, on the average,
it will be found that
9 are wildtype (light green).
3 are blue.
3 are yellow.
I is white.
It will be agreed that the wildtype class (or
phaenotype) will include the following genetic
constitutions (genotypes):-
+
+.
+
+.
the "yellow"
chromosome
pair
the "blue"
chromosome
parr
S·quares
~+~+ ~+~+
1
~+~+ ~+~b
2 5
Total
bght green
2
light green
heterozygous
for blue
32
~+ ~Y ~+ ~+
~+ ~Y ~+ ~b
3 9
4 7 10 13
2
light green
heterozygous
for yellow
4
hght green
heterozygous
for both
yellow and
blue
9
Fig 10
for the reason that the normal alleles of blue
and of yellow are dominants. Only I in the 9
is doubly homozygous and therefore truebreeding.
It will be agreed that the blue phaenotyp(, will
include the following genotypes (a blue bird
must be h01l)ozygous for blue).
Squares
Total
11
1
homozygous
blue
12 15
2
homozygous
blue
heterozygous
lor yello1f
~+ ~+ ~b ~b
3
Fig. 11.
It wjl1 be agreed that the yellow phaenotype
will include the following genotypes (a yellow
bird must be homozygous for yellow).
Squares
Hy
~+ ~+
(j
~ ~Y
~+ ~b
S 14
y
33
Total
homozygous
yellow
2
homozygous
yellow
heterozygous
for blue
3
Fig 12,
We are now left only with square 16 with a
genotype~ ~; that is, the double recessive.
This
is the "white" of the fancy. There is no white
gene yet known, the white is due to the interaction of two genes, the blue and the yellow,
both of these being present in duplicate.
The 9 : 3 : 3 : I ratio is typical of the F2 of a
Mendelian dif?ybtid experiment, i.e. of an experiment in which the PI individuals differ one from
the other in respect of two members of two pairs
of allelomorphic characters; in this case one
parent was blue and not yellow, and the other
was yellow and not blue.
Now that we know what the white of the
budgerigar is genetically, let us mate white to
yellow, to blue, to wildtype green, and also to
the PI doubly heterozygous wildtype of the
last experiment.
34
the "yellow'
chromosome
pair
y
(1) whit
y
the 'blue'
chromosome
pair
b
the 'blue'
chromosome
pair
the 'yellow'
chromosome
paLr
+
+
Y
X
b
Y
Y
yellow
b
Y
+
yellow, heterozygous
for blue
So white to yellow gives all yellows for the
reason that white is homozygous for yellow.
y
(2) whlt·e---
y
b
+
---x--b
+
Y
b
blue
b
b
+
b
blue heterozygous
for yellow
So white to blue gives all blues for the reason
that white is homozygous for blue.
y
b
+
+
(3) white - - y
---x--b
+
Y
b
+
+
---wtIdtype
green
+
wlldtype green heterozygous
for yellow and blue
So white to wild type (homozygous) green gives
all wildtype greens for the reason that the
wildtype alleles of blue and of yellow are dominant to their respective recessive mutant alleles.
3j
y
(4) whlte
y
y
b
b
- - x -+b
Y
b
+
b
-- : --
+
+
wild type green
blue
heterozygous heterozygous
for yellow and
for yellow
blue
25 per cent
25 per cent.
y
Y
Y
+ wlldtype green
- - heterozygous
b for yellow and
blue
b
+
Y
b
--F)
b
Y
yellow
heterozygous
for blue
whIte
25 per cent
25 per cent.
Lest this exercise should prove somewhat
difficult, let us state it so:-
x
Fig. 13
Into each gamete of the white bird there can
pass one member of each chromosome pair.
Since the members of a pair are alike in respect of
the genes they carry, it matters not which passes.
Each gamete will possess one blue chromosome
with a blue gene, and one yellow chromosome
with a yellow gene.
But, as we saw in the last exercise, the doubly
heterozygous wildtype green elaborates four
.
kinds of gametes.
Take the yellow chromosome A of the white and
combine it with the yellow chromosome C ofNo.
I gamete provided by the doubly heterozygous
36
wildtype. Then take the blue chromosome B
of the white, and alongside it place the blue
chromosome E of the wildtype gamete No. 1.
You now have an individual with two yellow
chromosomes and two blue chromosomes. One
of the yellows carries the yellow gene, the other
the wild type allele of yellow; one of the blues
carries the blue gene, the other the wildtype
allele of blue. This bird is a wildtype for the
reason that the yellow gene and its wildtype
allele and the blue gene and its normal allele
cancel each other out.
In the same way build up individuals by
combining the gametes of the white with the
other three gametes of the wildtype, a~ ways
putting the yellow chromosomes first and together, and the blue chromosomes second and
together.
gametes of
the whlte
gametes of the wildtype green heterozygous
for yellow and blue
234
A B
C E
C F
AC BF
iA C B E
wildtype green
blue
heterozygous for heterozygous
yellow and blue
for yellow
25 per cent.
25 per cent.
D E
ADBE
yellow
heterozygous
for blue
25 per cent.
FIg. 14
D
F
A D B F
whIte
25 per cent.
37
This last mating is the method employed to show
that the gene pairs involved in an experiment
are resident in different chromosomes. It is
because the genes for blue and for its wildtype allele
reside in 011e pair of chromosomes whilst the genes for
yellow and for its wildtype allele reside in a different
pair that this backcross of the doub!J heterozygous
dominant to the double recessive gives four phaenotypes
itt equal !lumbers.
D
III
GREYWING
T is enough for our present purposes to state
that the gene is to the geneticist what the
atom or the proton are to the physicist and
chemist-a thought model of the elementary unit
of which larger and more complicated integrated
things are built. They are the agents that, it is
assumed, launch the processes of development
and differentiation, the determinants which decide
from the beginning much of what the individual
shall be. They can be thought of as being strung
like beads upon a string along the length of 1 he
chromosome. We know definitely that these
units are resident in the cl~romosomes, each in
its own particular place or loetls on a particular
chromosome. If the chromosome is lost, the
gene is lost with it, and this will be shown in the
explanation of the bicoloured bird or halfsider.
A "sport" happens when a gene mutates; th:1t
is, when it changes in some way as yet unknown.
Thus a certain kind of change, a certain mutation,
occurred in the gene that we now know as the
wildtype allele of blue, and it then became the
blue gene. A mutation in the normal allele of
yellow produced yellow. A mutation results
from a certain specific kind of change in the
organisation of a particular gene, and when such
a mutation has occurred it persists so long as
individuals possessing it continue to be born,
and it will obey all the laws of organic inheritance.
I
38
GREYWING
39
A mutation is not the loss, the absence, of something; it is merely a different state of something
that has previously. existed. The relation 6£ a
mutated gene to its normal fellow or to other
mutated forms of the same gene is known as
allelomorphism.
The word "allelomorphism" merely means
"another form" or a "different form." The
genes exist in pairs and each gene has its partner,
as each chromosome has its partner. Moreover,
each gene has a definite position on its chromosome which no other gene can occupy, and the
two genes that form a pair are normally identical
in nature and function, i.e., are homologous. But
when a gene mutates it becomes different in
some way from its partner, though it still occupies
the same position. - And now the pair are no
longer identical: we may say there are now two
forms of the gene occupying that position: the
old or normaL wildtype form on one chromosome,.
and the new or mutant form on the partner
chromosome. The old form we call the normal
or wildtype .allelomorph or allele; the new form
we call the mutant allelomorph or allele. By
appropriate mating we may bring together in the
same bird either an old and a new allele, or two
of the old normal ones, or else two of the new
ones. ff7hen bo'th alleles are the same, either both
old or both new, we sqy the ·bird is homoz)'gous for
that allele and for the character associated with it.
When it carries tlJ!O different alkles, it is said to be
heterozygous for each of them. Thus a blue bird is
said to be "homozygous for blue" when it
carries the gene which is responsible for its
L
40
blueness in double dose. But a green/blue bird
is only heterozygous for blue: it carries one
normal gene which allows it to be green, and
one mutant gene for blue. Since the normal
allele is dominant to the mutant, the bird is
green. A cobalt is a heterozygous Dark bird.
All "split" birds are heterozygous for somethiNg.
But a gene may mutate more than once at
dilferent times: it may evolve several different
forms of itself. Thus a certain wildtype gene
once mutated so as to produce a yellow bird.
Years later the same gene mutated and produced
a greywing bird. So here we have three different
alleles which may occupy this particular pLl.ce
on a certain pair of chromosomes. But since on
a pair of chromosomes there is only room for
two alleles, we cannot bring them all together in
the same bird. Four or five or more alleles may
arise in this way, for some genes mutate more
often than others, but there is never room for
more than two alleles on a pair of chromosomes.
Each individual possesses its genes in pairs.
So that, in respect of the genes y yellow,
the wildtype allele of
,gr greywing, and
yellow and/or greywing, anyone individual can
possess one of the following pairs. (The yellow
chromosome alone is concerned.)
+
+
y
~
+
+
y
+
y
~
y
gr
~
But no bird can be Homozygous for both yellow
and greywing, and so there is no greywing yellow
that can breed true. There can be homozygous
wildtype, homozygous yellow, and homozygous
41
GREYWING
greywlllg, but on one and the same yellow
chromosome there can be either the gene for
greywing or else the gene for yellow, but not
both.
The greywing-yellow compound bird
illustrates the fact that when different alleles are
present together commonly one can behave as a
dominant to the other, though in other cases an
intermediate effect is produced.
The following is a complete list of the possible
combinations of greywing, yellow, and their
normal allele:Genetzc
formula
+
+
+
Genetzc descrzptzons
Fancy names
wlldtype
lIght green
heterozygous yellow
green/yellow
yellow
lIght yellow
heterozygous greywmg
green/greywmg
greywmg
greywing hght green
greywing yellow compound
greywmg green/yellow
y
y
y
+
gr
gr
gr
y
gr
IV
LINKAGE OF BLUE AND DARK
OU will have noted that we have not
inquired as to the manner in which the
gene is related to its end-result, the character.
It matters not at all for our present purposes
what exactly the blue gene does, or how it does
it; it is sufficient that we should relate the
character blue plumage colour to a particular
gene, the "blue" gene. There is no need for us
to speculate concerning the kind of fat and the
sort of enzyme that are involved in any pl- ysiological processes within the body. Genetics,
strictly, is not concerned with developmental
physiology; it deals with the analysis of the
genotype by means of a study of the phaenotype;
it deals with the beginning and the end, the gene
and the character. So far we have learned two
things: (I) that the genes for characters that show
independent assortment and recombination are
resident in different chromosomes. The evidence
for such independent assortment and recombination was given in detail in the F2 of the dihybrid
experiment (blue and yellow) and also in the
backcross between the double recessive white
(homozygous blue yellow) and the doubly
heterozygous Fl wildtype green out of the blue
and yellow mating. (2) That a wildtype gene
can mutate in more than one way to give two
distinct mutant genes yielding two distinct
mutant characters, and that the genetic relation
Y
42
43
of these alleles is such that no bird can be homozygous for any two of them. Thus, though
there can be true-breeding yellows and truebreeding greywings, the greywing yellow is a
compound that cannot breed true; it is always
heterozygous, and half of its gametes will carry
the yellow gene, the other half the greywing
gene.
,
We must now return to a further consideration
of the gene which we have called Dark, the gene
that modifies the action of the wildtype, the
yellow and the blue genes, turning light green
into dark green or olive, skyblue into cobalt or
mauve, and light yellow into dark yellow or
yellow olive. It possesses considerable interest
for several reasons. One is that one dose of it
yields an effect halfway between the double dose
and its absence. Furthermore, it is linked with
blue; that is to say, the gene D is on the same
chromosome as is the blue gene.
You will have appreciated the reason why
Dark greens, cobalts, and Dark yellows are
incapable, of breeding true. They cannot, because
they are constitutionally heterozygous; their D
gene is present in the simplex state, and so only
half their gametes will carry it. So dark greens
will inevitably produce, when mated inter se,
light greens, dark greens, and olives in the ratio
I : 2 : 1. Cobalts will yield skyblues, cobalts, and
mauves in the ratio I: 2 : I; and dark yellows
will yield light yellows, dark yellows, and
yellow olives in the ratio I : 2 : I. You will
note that this I : 2 : I ratio is the typical F2
ratio of a monohybrid mating; it is the same as
44
the 3 : I ratio. The difference between a I : 2 : I
and a 3 : I ratio is due to the fact that the Fl
individual, the heterozygote that is, has a character
all its own. When you see a dark green, a cobalt,
or a dark yellow, you know at once that it is
heterozygous. The typical 3: I ratio itself is
really a I : 2 : I, for it includes I homozygous
dominant, 2 heterozygous dominants, and I
recessive. So
b
+
b
+
cobalt
+
b
b
+
skyblues
25 percent
I
- - - - x _ _b
D
b
D
b
+
b
D
b
D
b
cobalts
50 per cent.
2
+
cobalt
D
b
b D
mauves
25 percent
F,
I
This explanation takes it for granted, as you will
observe, that the two genes on one and the same
chromosome (the "blue" chromosome) remain
together as the chromosome, the vehicle that
carries them, is transferred from generation to
generation. It postulates, that is, that the genes
are and remain linked.
Breeders have known for a long time tr.at
there was some essential difference between the
relation of blue and Dark on the one hand, and
either of these and yellow on the other. They
distinguish two kinds of Dark green budgerigars
which are 4eterozygous for blue. Both of these are
alike phaenotypically, that is are indistinguishable
45
by the eye; they are to be recognised only by
the different modes of segregation of the blue
and Dark genes among their offspring. Birds
of Type I Dark green produce, when mated to a
skyblue, offspring of two kinds: Dark greens and
light blues. Whereas the Type II Dark greens,
mated similarly, yield light greens and Dark
blues.
A Type I Dark green can be produced in the
following manner. An olive is mated to a skyblue,
and only Dark greens are produced, and these
will be Type I birds, for if they are mated to
skyblues, they will produce greens that are
Dark and blues that are light. This is completely comprehensible if, in the olive parent,
D and the normal allele of blue were on the same
(the "blue") chromosome. So:-
+
olrve
D
+
b
---X---+ D
b
+
+
D
Dark green (Type I)
b
skyblue
+
X
backcross
+
D
b
+
b
+
b
+
Dark green
skyblue
Type II Dark greens are produced when a light
green is mated to a mauve, and when the FI
Dark greens are mated to skyblues they give
light greens and Dark blues. So:-
46
lIght green
+ +
+ +
b D
---- X ---
Dark green (Type II)
light green
b
mauve
D
+ +
b
b
b
+
+
b
D
- - - - - X ------D
+ +
b +
skyblue
----cobalt
b +
(Dark blue)
There is no other possible explanation of the
difference between these results save that in the
two Pl Dark greens the Dark gene is present in
different company. The combination band D :J the
hall-mark oj the Type II Dark grem; the combination
and D that oj Type I.
+
Here, then, is further proof that a study of
the progeny reveals the genetic constitution of
the parent. In these experiments, of course, we
knew the genotype of the sky blues, for to be
skyblue at all they could not carry the Dark
gene.
You will remember that the test for independent assortment and recombination (that is to
say, the proof that two genes were residenl in
different chromosomes) was the production- of
four phaenotypes in the backcross of the doubly
heterozygous individual to the double recessive.
A light green carrying hidden a gene for blue
and a gene for yellow, when backcrossed to the
double recessive white, gave 25 per cent. wild-
47
type; 25 per cent. blue; 25 per cent. yellow; and
25 per cent. white.
The equivalent test for a proof of linkage is, as
you see, the appearance of two phaenorypes instead of
four in this backcross. A Dark green, heterozygous
for the dominant' Dark gene, and also carrying
blue, when mated to the double recessive, gives
50 per cent. Dark greens and 50 per cent. light
blues, or else 50 per cent. light greens and 50
per cent. Dark blues. It is because two phaenotypes and not four appear that we know that
we are dealing with linked genes.
So we arrive at the cQnclusion that the genes
are not transmitted singly but in groups, and
genes belonging to the same group, being on the
same chromosome, segregate together. Linkage
of this kind can only be observed between two
genes when two different alleles 'of each of them
are present in the same individual. As you will
have seen, both types of the Dark green are
doubly heterozygous. Each has one blue gene,
one normal allele of blue, one Dark gene, and
one normal allele of Dark. When this condition
is satisfied and the appropriate cross is made, it
is only necessary to observe whether there is free
assortment and recombination between the members of one pair with both those of the other,
or whether one member of one pair segregates .
with one member of the other pair more often
than not. In the latter case the genes are linked.
Both types of Dark greens can be produced
by the mating Dark green (not carrying the blue
gene) X cobalt; so:-
48
+ Db])
+ +
b
+
------- X -------
Dark green
cobalt
0
+
+
D
+ +
D
b +
Dark green
Type I
b D
ohve
b D
Dark green
Type II
+ +
b
+
Fl
hght green _
The difference here is that Type I receives its D
gene from the green parent in association with
the wildtype allele of blue, whereas Type II gets
its D from its cobalt parent in association with
the blue gene.
There is no suggestion of linkage bet ween
Dark and yellow, nor would any be exp~':ted
since we know that blue and yellow are not
linked. Thus, olive X light yellow gives all Dark
greens which, when mated back to light yellow,
give Dark greens and light greens, Dark yellows
and hght yellows in equal numbers.
+
+
ohve
D
Y
D
Y
--x
+
Dark green
y
+
+
+
D
Y
- X-
-
+
+
y
back-cross
light yellow
or
ABC
D
E
hghtyellow
F
49
A C
A
AECF
Dark greens
25 per cent
B
D
C
B
AEDF
lIght greens
25 per cent.
E
D
F
BED F
light yellows
25 per cent.
BECF
Dark yellows
25 per cent.
FIg 15.
Similarly, yellow olive mated with light green
gives all Dark greens, and these, mated to light
yellows, give the four possible types in equal
numbers.
y
-
yellow ohve
D
X -
D
y
I
Y
Dark green
+
-
+
+
D
+
+
+
X -
Y
y
WIld type hght green
+
+
hght yellow
or
A B
A
CAD
C
~
D
B C
B
E
D
F
E F
~o
A E C F
Dark yellows
25 per cent
A E
D F
BEe F
light yellows
Dark greens
25 per cent
25 per cent
FIg 16
BED F
lIght greens
25 per cent.
From these results we know that yellow and
Dark are not linked, that the genes reside in
different chromosomes.
But, like most other things, linkage is not
always perfect. Exceptionally, the Type I Dark
green, when mated to a skyblue, gives in addition
to the usual Dark greens and light blues, a light
green or a Dark blue. Since this can and does
happen, it demands an explanation. The 1 ype I
Dark green, we agreed, had the constitution
+
D
b
+
Let us represent this formula in terms of chromosomes, one member of the blue chromosome
pair carrying the normal allele of blue and D,
the other member carrying the blue gene and
the normal allele of D. If we mate this Dark
green to a skyblue, so:Dark green Type I
+
D
b
+
---x--b +
b +
svyblue
we get, as the result of this mating, two phaenotypes: Dark
=.9-~light blues bb + for
~~~\..~""'HhES ....
+
the reas ~fl'"J.riIm1'it~~~arent elaborates
gametefla!fift a kind (with"rt;tJ~e gene and the
5I
normal allele of D), the Dark green parent
elaborates two kinds in equal numbers (+D) and
(b+).
The number of the phaenotypes is
determined by the number of the different kinds
of gametes elaborated by the doubly heterozygous
parent. Obviously, we cannot get light greens
and Dark blues out of this mating so long as
the normal allele of b and the Dark gene on the
one hand, and the blue gene and the normal
allele of D on the other, remain associated. To
get a light green and a Dark blue we must produce
birds with the following formulre:b
D
+ +
lIght green
b
+
b
+
Dark blue (cobalt)
The probl~m, then, is to bring about an association between the b and the D genes in the
gametes of the Dark green parent, and at the
same time an association between the normal
alleles of band D. In other words we have to
manufacture four kinds of gametes instead of
two; so:-
'-----v-----'
'--y---J
the two kmds of gametes
normally produced
the other two kmds that
must be produced If lIght
greens and Dark blues are
FIg 17
to appear
This can be done if ,the/-place where the blue
gene resides in the ~hrprnb~ame.is some distance
away along the lel1gih "orth"e thfOmpsome from
the place where ·'th'e Dark gene is' to be found,
5Z.
..Q
+
--.---r..0
+
..0
--
..00
_Q
X
~ _::,
~
...0
+ t
-
'"
ClJ
:>
0
M
S
if)
+
Ul
0
h
I
U
UJ
¥
bD
-\-
+ c:l
+
I
0
...a +
+
~~
~
....0
t
_c
+
...0
+
+
0
~
~
3
and if between these places the two chromosomes
should stick, break, and reunite in such a way
that the top part of one joins up with the bottom
part of the other. This is actually what does
happen, and when it happens there is an interchange of material between the two members of
a homologous pair of chromosomes, and in the
case where there are two pairs of genes involved
and where the individual is heterozygous in
respect of each of them, new linkage relations
become established due to crossing-over (Fig. 18.)
The results of a great number of such matings
show that in such cases there are to be expected
about 86 per cent. of Dark greens and skyblues,
these appearing in equal numbers, and about
14 per cent. of light greens and co balts, these
also appearing in equal numbers. Crossing-over
cannot be regarded as the result of a fault in the
chromosome distributing mechanism, though
when one considers the delicacy of the mechanism
and the rapidity with which it works, it is
surprising that faults are not more common.
The fact that we can always expect to get about
14 per cent. of recombination classes as a result
of the. above mating shows that crossing-over is
not merely an accident. It is as though the
construction of the chromosomes is such that
if they must be subjected to this twisting and
sticking, they must necessarily break in a certain
,percentage of instances.
You will note that the four classes: Dark green,
light green, light blue, and Dark blue, are
exactly the classes you would expect to get if
E
54
Dark and blue were not linked at all. Four
classes among the progeny of such a mating as
this is typical of independent assortment and
recombination, but note: the four classes do not
occur in equal numbers, light greens and Dark
blues occur only exceptionally, and linkage is
the rule. Though for our present purposes it is
of no great importance, it may be stated that the
fact that the crossing-over value (CO.V.) between
blue and Dark is 14 per cent. is taken to mean
that the positions (the loci) in the chromosome
occupied by these two genes respectively are
separated one from the other by a distance equal
to about one-seventh (14 X 7=98) of the total
length of the chromosome. The idea 1ehind
this is that if a pair of homologous chromosomes
can stick, break, and rejoin as easily at one level
as at any other, then the further apart two points
are along their length the more frequent will be
crossing-over between them, and thus the frequency of crossing-over is a measure of this
distance.
It will have been understood, of course, that
crossing-over disturbs genetic relationships only
when at least two mutant genes as well as their
alleles are involved. Obviously, if in an exchange
you receive exactly the equivalent of what you
give, nothing is gained and nothing is lost;
everything remains as it was before.
V
CINNAMON
A
NOTHER kind of linkage remains to be
discussed: sex linkage. Male differs from
female in a multitude of ways: cere colour,.
length of tail, behaviour. The male is equipped
for the production of spermatozoa, the female
for the production of eggs, and the sexual and
reproductive habits of the two sexes are different.
But deeper than all these is yet another difference.
Under the micro~cope it is possible to tell whether
the tissue one examines is from a male or a female
by reference to differences in chromosome
constitution.
You have learnt that in both male and female'
the chromosomes exist in the form of a definite
number of pairs of homologous chromosomes,
and that in the case of all these pairs, save one,
the chromosomes of male and female are identical,
but that in the case of this particular pair they
differ. In the coc~ this pair consists of two equal
and similar mates: in the female, one member of
this pair is like the two members of the pair in,
the male, but its mate is small, unequal, dissimilar.
Because in respect of this pair of chromosomes
the sexes differ, these chromosomes are called
the sex chromosomes, and further, to distinguish
between them the two in the male and the one
like them in the female are known as the Xchromosomes, whilst the small unequal mate of
55
56
the X in the female is known as the Y -chromosome. The chromosomes other than the sex
chromosomes are, in both sexes, called the
autosoJnes. So male can be distinguished from
female for the reason that he has two X-chromosomes whilst she has but one.
Now, if there are any mutant genes on the
X-chromosome, it follows that in respect of them
the female must be constitutionally heterozygous;
that is to say, only half her eggs will carry these
genes. Cinnamon in the budgerigar has been
shown to be a sex-linked character, as it is also
in the canary. In respect of the gene for this
character there can be three kinds of cod:s but
only two kinds of hens. So (the X-chrom0some
alone is shown):-
+
+
en
en
+
en
eoeks
+
en
hens
The cock can have no cinnamon gene, one
cinnamon gene, or two, but he will be a cinnamon
only if he has two, for cinnamon is a recessive.
If he has only one cinnamon gene he will be a
carrier of the gene, but will not exhibit the
character. But the hen is either cinnamon or
else she is not; she cannot be a carrier. She has
only one X -chromosome, and apparently if there
are genes on her Y -chromosome they do not in
any way affect the action of the genes on her
X-chromosome. The Y can therefore be disregarded. So if a hen is cinnamon, she carries
the cinnamon gene: if she is not cinnamon, then
she does not possess the gene.
CINNAMON
57
If we follow the distribution of the cinnamon
character through a number of generations of an
experiment, we see the reason why this gene is
placed upon the X-chromosome and not upon an
autosome like blue or yellow.
It becomes
obvious that the mechanism that is distributing
this gene is also concerned in the determination
of the sex of the individuals.
You have learnt that a cock is XX, i.c. has in
addition to a considerable number of autosome
pairs, a pair of sex chromosomes, a pair of X's,
and that the female, instead of two X's has an
XY pair. You have also learnt that into each
ripe sperm one member of each pair of chromosomes, autosomes, and sex chromosomes alike,
passes. So each sperm will include an X-chromosome! But for the same reason there will be two
kinds of eggs, one with an X-chromosome and
one with a Y .. Fertilisation thus comes to mean
the meeting of an X with an X, or else of an
X with a Y. The first will give an XX individual,
a cock; the second an XY individual, a hen.
Thus, if the X and the Yeggs are produced in
equal numbers there should be equal numbers
of males and females produced by the mating of
a male and a female. This is the case if we
consider a sufficient number of matings. Thus.
the sex-chromosome distributing mechanism is
the sex-determining mechanism. For the production of males and females all that is wanted is
that one sex should, in respect of the elements
of the sex-determining mechanism, produce two
kinds of gametes, male-determining and femaledetermining respectively, whilst the other sex
58
should produce but one kind of gamete. In the
budgerigar it is the female that produces the two
kinds of gametes; in the case of man it is the male.
Next let us consider the implications of the
suggestion that the cinnamon gene is resident in
the X -chromosome. Let us mate a wildtype cock
to a cinnamon hen (only the X-chromosomes
are shown):wildtype cock
+
+
en
+
+
-- X
cmnamon hen
Wlldtypesons(carriers)-- X - - wlldtype aaughters Fl
en
+
+
en
wIidtype cock
wIld type cock
(carner)
+
+
ell
----
wIidtype hen
cmnamon hen
F.
These results can be stated so: In the case
of. the mating of a hen exhibiting a sex-linked
recessive character, with a wildtype male, the
sex-linked recessive character reappears in 50
per cent. of the granddaughters. You note that
the .F2 3: I ratio is typical of the Mendelian
monohybrid experiment; the normal allele of
cinnamon is dominant to cinnamon. But in
order to state the results obtained it is necessary
to refer to the sex of the individuals as well as
to the characters they exhibit; the recessive sexlinked character of the Pl female is exhibited by
half the females in F 2 • You note that not all the
CINNAMON
59
females in F2 are cinnamons, but that all the
cinnamons are females-the sex of these can be
recognised from their plumage character.
Now consider the reciprocal cross, cmnamon
cock to wildtype hen:cn
CInnamon cock
cn
X
cn
wIldtype sons
(earners)
,
en
+
en
en
cInnamon cock emnamon hen
+
wIld type hen PI
cn
Clnnamon
daughters
X
+
+
en
wIldtype cock
(carner)
emnamon hen
FI
This is an example of what is known as criss-cross
inheritance: In the FI generation the sons have
the character that the mother exhibited, the
daughters that of their father. In the F2 the hens
are either wildtype or cinnamon, the cocks either
cinnamon or carriers, and the four classes occur
in equal numbers.
It will be agreed that when a mutation, as it
often does, occurs during the development of
the gametes, the individual to whose making
this gamete went will possess the mutant gene
only in the single dose. Now, if this new gene
happens to be on an X -shromosome, it follows
that if the individual is a hen she will exhibit the
new character, the mutation is revealed at once.
But if it is a cock that receives the new mutant
60
THE GE)'JETICS OF THE BUDGERIGAR
gene in this way, the new character will not
appear, if the new gene is recessive to the wildtype
allele in the other X-chromosome. Such a cock
would certainly be mated to a hen carrying the
wildtype allele of the new sex-linked recessive
mutant gene, and then the new mutant character
would appear in half his daughters, and half his
sons would be carriers like himself. Call the new
gene a:-
+
+
x
a
+
+
+
a
a
+
Since it is impossible, let us assume, tu distinguish between the
!
and the : sons, the next
step would be to mate one of the daughters
with the new character back to her father, when
half the offspring, both males and females,
would be expected to exhibit the new character.
So:+
a
father
--X--
a
+
a
+
II
a
daughter
back-cross
a
a
So that, to get birds of both sexes with a new
sex-linked recessive character is a fairly simple
matter. It is not so simple in the case of an
autosomal recessive for the reason that in both
CINNAMON
61
sexes the heterozygote exhibits the dominant
character of a pair. It is to be assumed that a
gene mutation, when first it occurs, is to be found
only in one individual, and in the single dose in
that individual. Therefore such a bird cannot be
mated to its like but only to such as possess the
wildtype allele of the new gene. The result is
the production of a few more heterozygotes.
Before the recessive character appears two such
heterozygotes must have been mated, and since
the breeder decides which bird shall mate with
which, and since he cannot suspect that a new
recessive mutation has occurred in his stock, it
may be many years before the mutation is revealed.
In such a case the mutation may have occurred a
year or two before it became visible. Of course,
if the mutation is a dominant it is revealed in the
first individual to possess the new gene. Recessive
mutations are probably not nearly so rare as
they seem; they occur but are lost or never
revealed for the reason that most breeders of
budgerigars raise relatively few birds, and by
introducing new stock birds always maintain the
mutant gene in the heterozygous state. The
real recipe for the revelation of new recessive
autosomal mutations is to keep larger stocks
and to in-breed consistently. In-breeding is
the surest and quickest way to uncover such
mutants, for by such a process the proportion
of homozygous individuals in the stock is
continually increased, and before an individual
can exhibit a recessive character it must
possess the gene in duplicate; it must be
homozygous.
6z
We have now called upon three chromosomes
(three pairs, that is) in our interpretation of the
facts of heredity in the budgerigar. It is a
convention always to call the X -chromosome
Chromosome I. Since yellow is the oldest of
the mutant forms, we will call the yellow chromosome, that is the chromosome in which the
yellow gene, the greywing gene, and their normal
alleles reside, Chromosome II. The blue chromosome, that in which are to be found the blue
gene, the Dark gene and their wildtype alleles,
shall be Chromosome III.
Using the same
convention, the sex-linked group of hereditary
characters is Group I; yellow and all characters
linked with yellow and with each other constitute Group II; blue and all characters linked
with blue and with each other form Group HI.
Since there are some 20-30 other pairs of homologous chromosomes, there can be some 20-30
more linkage groups, so that the genetics of the
budgerigar is as yet by no means complete.
It would be laborious, and surely it is unnecessary, to provide a complete list of the genetic
constitutions of the different homozygous and
heterozygous forms of the budgerigar. It will
be agreed that a wild type light green can carry
any or all of the recessive mutant genes in the
single dose; it can be heterozygous for cinnamon
(if a cock), yellow, greywing, and blue, since
none of these genes, in the heterozygous condition, disturbs, to any extent, those developmental
processes which yield the wildtype characterisation. We say to any extent for the reason that
it is not improbable that if these heterozygotes
CINNAMON
were examined very carefully some detectable
difference between the homozygous and heterozygous light green would be revealed. It is not
impossible, we may assume, that the specialist
in light greens can distinguish between the
homozygous light green and the light green
carrying blue.
For exhibition purposes it may be desirable to
introduce one or more recessive genes in the
single dose to modify the general ground colour,
but the breeder should always know what the
constitution of his stock actually is, and should
be in a position to tell any intending buyer of
his birds what exactly he is buying, for undoubtedly the purchaser wants to know what
recessives he is 'introducing into his own stock.
Possibly in the future the advertisements for
budgerigars will be something like the following:
+ + bD
+ + b +
For sale or, \Vanted two cocks -+yb+
cny++
A light green can carry any of the recessive
genes: the blue must be homozygous for blue,
the yellow for yellow, but they can carry any of
the other recessives, and the breeder should
know which his birds carry, for his pedigrees
Light greens that throw an
will tell him.
occasional white must be heterozygous for both
yellow and blue. Yellows that throw an occasional
white must be heterozygous for blue. Blues
that throw an occasional white must be heterozygous for yellow, and so on. A hen cannot be
a split cinnamon. There is no mystery or deceit
about these things, but only misunderstanding
and carelessness.
VI
THE BI-COLOURED OR HALFSIDER
S
TRICTLY used, the term bi-coloured is
applied only to what fanciers know as a
halfsider, a bird with the plumage of one
half of the body (one side from head to tail
inclusive) of a colour appropriate to one variety,
whilst the other half (the other side) is of a colour
characteristic of another variety. But the term
is often used more loosely to describe a bird
with two different varietal colours in its plumage,
the actual regional distribution of these being of
no account.
Quite a number of such birds have' been
recorded in the literature, and it is generally
accepted that' they have had their origin in an
egg with two germs. It is established in other
forms of life that such an egg can and does
exist. It can arise in one of three ways:I. A sperm enters an egg which has already
begun its first division and fuses with one of the
resulting nuclei, whilst the other develops
independently.
2. Two spermatozoa enter one and the same
egg; one of them fuses with the egg nucleus
whilst the other develops independently.
3. The nucleus of an egg divides to give rise
to two; two sperms enter this binucleate egg and
one sperm fuses with each of the egg nuclei.
Probably these somewhat bald statements
64
65
demand some elaboration. Fertilisation normally
consists essentially in the fusion of the nucleus
of the sperm with the nucleus of the egg, each
with the half or haploid number of the chromosomes, to form the fertilisation nucleus in which
the full number of the chromosomes is restored.
One member of each pair of homologous
chromosomes has come iNO this fertilisation
nucleus by way of the sperm, the other by way
of the egg. This fertilised egg then proceeds to
divide. The wall of the nucleus disappears, the
chromosomes come to lie along the equator of
the cell, and each divides along its length into
two. The cell itself begins to divide at its equator,
the half chromosomes separate and move towards
the opposite poles of the cell. When the division
of the cell is complete, two daughter cells have
been formed, and in the nucleus of each there is
the full n'umber of chromosomes, and each of
these has been derived from the equivalent
chromosome in the fertilised egg. Cell division
continues, each of these daughter cells divides
into two, and again each of the resulting cells
divides into two, and so on and on until the
whole body is formed. Of course, this process
IS nothing but growth, but in addition to this
increase in cell number, this increase in size,
another process is pursued, that ot differentiation:
whereby cells come to differ among themselves
to form different tissue,S with different functions. Moreover, growth is not equal in all
parts of the developing embryo so that the
body becomes fashioned into the likeness of the
species.
66
THE GENEl'ICS OF THE BUDGERIGAR
Now, in the insect and the bird it seems that
from one of the two daughter cells produced by
the first division of the fertilised egg one side of
the body is developed, and that from the other
daughter cell the other side of the body is
derived. Normally, for the reasons that each of
these two daughter cells has the full number of
chromosomes and that of each homologous pair
of chromosomes one member has come from the
father, the other from the mother, the two sides
of the body are alike.
But if the egg nucleus has divided before the
sperm has entered the egg, and if the spam
nucleus fuses with one of the daughter nuclei
whilst the other daughter nucleus remains llnfertilised and yet can divide and give rise to
cells, it will be agreed that the cells of one side
of the body can have the full chromosome
complement whereas those of the other side will
only have one member of each homologous
pair and this will have been received from the
mother. Thus, if the father of a bi-coloured bird
was light green and the mother a skyblue, let us
say, and the halfsider itself was light green on
the right side and skyblue on the left, we know
that the right side was green because in the case
of the blue chromosome pair, there was the wildtype gene on one member and the blue gene on the
other, and wildtype is dominant to blue. The
left side was skyblue because there was only one
member of each chromosome pair, and therefore
only one blue chromosome and this had a blue
gene on it. In the absence of its wildtype allele
the recessive blue would be expressed (Fig. 19. I).
67
The same explanation could be applied to a bicolour due to the presence of two sperm in one
egg, if the father were skyblue and the mother
light green (Fig. 19. 2).
The argument would demand a slight modification if, for the explanation of a skyblue-light
green halfsider, we wish to invoke the double
fertilisation of a binucleated egg. One of the
parents of the bi-coloured, the mother, would
need to be skyblue, the other, the father, would
have to be a heterozygous light green (carrying
blue). Both of the egg nuclei would include a
chromosome carrying the blue gene; of the two
sperms one would carry the blue gene, the other
its wildtype allele; fertilisation would bring
together two blue genes to give a skyblue half;
in the other half a blue gene and its wildtype
a:lele would give a light green (Fig. 19· 3).
Fertilisation.
'·8~~
1
1
1
GG 88 88
1st division of the fertilIsed egg.
e=a nucleus (egg or sperm) includmg a chromosome carrying the wJ.!dtype
allele of "blue"
=a nucleus (egg or sperm) mcludmg a chromosome carrymg the "blue" gene
FIg 19.
o
68
These explanations are quite satisfactory if the
several assumptions which they involve are
warranted. Polyspermy (the entrance of more
than one sperm into an egg) is quite common
among birds, but it is not established that a
nucleus containing only half the proper number
of chromosomes can control the functions of
the cell fully and normally. It can and does in
insects and forms like these; the male bee, for
example, has only the half number, the haploid,
instead of the diploid number. But it has not
been shown that this is so in the case of the bird.
That each of the two halves of the body is
derived from one of the daughter cells formed
by the first division of the fertilised egg is also
not established in the case of the bird, but the
evidence that we are about to consider suggests
very· strongly indeed that dus is so. Again, it
can be shown to be the case in other and simpler
forms of life.
But these explanations, though they can
explain the haHsider, cannot explain a bird with
threequarters of its plumage, both hindquarters
and one forequarter, of a dominant varietal
colour, and the remaining forequarter of the
allelomorphic recessive colour, or one with a
small patch of a recessive colour, the rest of the
body being the allelomorphic dominant. And
such birds exist, and since they exist they demand
explanation. Now, it will be agreed that if there
can be found one explanation that can accommodate both the halfsider and the threequarterquarter and so on, it would be uneconomical
to disregard it and to devise two distinct
69
explanations to account for what appear to be
but different grades of the same phenomenon.
As it happens, there exists a perfectly satisfactory
hypothesis. It is known as the chromosome-elimination hypothesis, and, according to this, the bicoloured bird is a heterozygote; that is to say, in
respect of a contrasted pair of characters such as
light green (wildtype) and skyblue, it has on the
blue chromosome pair, Chromosome III, a blue
gene on one, and the normal allele of blue on the
other. It started its life as a fertilised egg which,disregarding all chromosomes except the third pair,
we will depict so (it receives one member of this
pair from its father, the other from its mother):-
~+ ~b
Fig. 20.
This fertilised egg proceeds to divide. First the
chromosomes take up a position at the equator
and each splits along its length, so:-
Fig. 21.
F
70
Fibres are seen running from the halved chromosomes to a star-shaped body near each pole of
the cell which becomes pinched around the
equator. The daughter chromosomes (the halves
of the split chromosomes) move away from each
other, just as though they were being pulled
apart by the retracting fibres. Normally they
should pass unhindered to a point near the star,
there to form the nucleus of the daughter cell.
But one becomes detached and is not included
in the daughter cell at all, so:-
Two daughter nuclei are formed, but whilst one
has two third chromosomes, one carrying the
gene for blue, the other its wildtype allele, the
other, baving lost the chromosome with the
wild type allele of blue, has only one third
chromosome, and this carries the blue gene. Of
course, all tbe other chromosomes are present;
this cell lacks but one member of the third pair.
71
From the daughter cell with both members of
the third pair one side of the body develops;
from the other with but one member of the third
pair, the opposite side of the body is derived.
Therefore, one side of the body will be wildtype,
light green, whilst the other will be skyblue.
You will note that the chromosome that is
eliminated must be the one carrying the dominant
gene of the pair concerned. If the father had the
dominant character, then it would be the paternal
chromosome that was lost. If the chromosome
with a blue gene had been lost, a halfsider would
not have resulted. The loss must uncover a
receSSlVe gene.
So, according to this explanation, the halfsider
must start as a heterozygote; the elimination of
the chromosome bearing the dominant gene
must occur at the first division of the fertilised
egg, and each half of the body must have its,
origin in one of the two daughter cells resulting
from this first division.
The three quarter-quarter bird can be as easily
explained.
Each of the two daughter cells
resulting from the first division of the fertilised
egg divides in its turn to give rise to two cells.
If the elimination of the chromosome occurs at
this second division, then the threequarterquarter results, for, from each of these granddaughter cells, as they may be called, one quarter
of the body is derived. And so you will see that
the later in development this elimination occurs,
the smaller will be the area that exhibits the
recessive character of the pair. If it occurs very
72.
late, then all that we should expect to find would
be a small clump of feathers of the recessive
colour. The extent of the recessive colour
provides us with a rough idea of the time during
development when the chromosome was lost.
But, it must be emphasised, the bird must be a
heterozygote; that is to say, one of its parents
must have had the dominant member of a pair
of contrasted characters, and the other must have
had the corresponding recessive.
Chromosome elimination is not the only
cause of this condition however. Mutation of a
gene can occur at any time, either during the
formation of the gametes or at any stage during
the development of the individual, it ca._n affect
either the germ cells o.r else the cells of the body;
that is to say, it can be either gametic or somatic.
If it occurs in a cell of the body, the gametes
are unaffected, and so somatic mutations are not
transmitted to the offspring. If it occurs in a
somatic cell from which an area of skin with its
feather follicles is derived, then the possibility
is created that this area will exhibit a plumage
colour character differing from that of the· rest
of the body. The mutation might of course
yield either a dominant or a recessive, and if the
former, the result of such mutation would be
visible in a heterozygous individual.
Non-disjunction (see page 76) following upon the
incomplete longitudinal division of a chromosome and a consequent failure on the part of
the two daughter chromosomes to separate could
yield one daughter cell with two and the other
73
with no representative of this particular chromosome carrying the dominant gene of a pair.
If this occurs late in development then it
becomes possible for a small area of the
body of a heterozygote to exhibit a recessive
character.
There is no reason to assume that the halfsider
will breed as a bi-colour, though there is evidence
which seems to indicate that this tendency to
eliminate chromosomes can be characteristic of
certain strains.
The bi-colour will breed either as a heterozygous dominant or else as a recessive. The male
has two testes, one on either side of the body;
the female has a single ovary on the left side.
But the sex gland of one side does not necessarily
arise from cells belonging to the same side; these
cells from which the sex glands are derived
migrate from the head region of the embryo
to the places where the adult sex glands are
found, so that they may be derived from cells
from either side of the body. In any case, the
cells of the sex gland, testis or ovary, can have
but one of two chromosome constitutions in
respect of Chromosome III in the halfsider.
There can be a pair of third chromosomes, one
carrying the blue gene, the other its wildtyp~
allele, or else there can be a single Chromosome
III carrying the blue gene.
So in the case of the cock with two testes,
one on the right, the other on the left, there
could be the following kinds of gametes elaborated:-
74
+
Both testes
and b=equal numbers of
gametes.
+ and b
Both testes b=all b gametes.
+
One testis
and b, the other testis b=
b gametes, but great excess of b.
+ and
In the case of the hen, the ovary must be
either
and b, or else b, and she will breed
either as a heterozygous dominant or else as a
blue. But in the case of the sex gland with only
one Chromosome III, half the gametes will be
lacking this chromosome, and it would not be
surprising to find that a bird breeding as a
recessive blue was markedly infertile.
+
These bi-coloured and parti-coloured buds are
what are known as autosomal colour mosaics
in order 'to distinguish them from other and
somewhat similar cases of bi-colourism due to
entirely different causes.
Elimination of the X -chromosome in the case
of a heterozygous cinnamon cock may also be
expected. The result, it may be surmised, will
be vastly interesting. If the X-chromosome
carrying the wildtype allele of cinnamon were
lost at the first division of the fertilised egg, it
would give rise, one may assume, to a halfsider,
cinnamon on one side of the body and wildtype
(i.e. not cinnamon) on the other. But since all
the cells and all the tissues derived from the cell
that had lost one X -chromosome would have
the constitution of a female, having only one
X-chromosome, it is not improbable that though
75
the sex glands were testes, as they could be, the
cere on the female side would not exhibit the
same intensity of colour as that on the male side,
and that there might be a difference in tail feather
length on the two sides. The actual kind and
degree of difference on the two sides of the body
cannot be stated, for as yet we do not know
anything about the influence of the testes and
ovaries respectively upon the plumage structure
and colour in the budgerigar. But since the male
and female are similarly coloured, it is not to be
expected that anyone will encounter a budgerigar
resembling those bullfinches, for example, which
have been described as having female colouration
on one side of the body and male colouration
on the other.
It is convenient here to refer to a phenomenon
that has puzzled many budgerigar breeders.
Can a budgerigar change its sex? The question
arises' for the reason that not infrequently an
aged and favourite hen has been known to assume
certain of the characters of the male. If such
specimens are dissected it is found that the
ovary is very small and imperfectly functional.
It would seem that the assumption of the male
!;:haracters by the hen is prevented by the presence
and action of the ovary, and if the ovary is
removed or atrophies in old age, these characters
then become expressed. It is not improbable
that sooner or later a case will be encountered
in which, following the destruction of the ovary,
testes have developed so that a hen has indeed
undergone complete sex transformation, even to
the extent of functioning as a male.
76
Sooner or later in the budgerigar, when the
cinnamonwing is as common as the greywing is
now, individuals will be encountered which will
seem to disprove the rules of sex-linked inheritance, and it will be shown that these
exceptional birds are the result of an abnormality
in the distribution of the X-chromosomes.
You will remember that the mating cinnam~n
cock and wildtype (non-cinnamon) hen gives
wildtype cocks and cinnamon hens, so:en
en
X
+
en
en
+
and that this was due to the facts that whereas
each sperm of the cinnamon cock include~ an
X -chromosome and that each of these carries
the cinnamon gene, there were two sorts of
eggs, one with an X-chromosome with the wildtype allele of cinnamon and the other a Ychromosome carrying neither the cinnamon
gene nor its wildtype allele.
There is a time during the formation of the
sperm when the unripe or immature sperm with
two X-chromosomes divides into two- and into
each of the resulting sperm there passes one
X-chromosome. That is to say, the two Xchromosomes disjoin, separate, as do also all the
autosome pairs. But suppose they didn't. Then
two spermatozoa would be formed, but one would
contain two X -chromosomes and the other would
contain none. (Of course, both of them would
contain the haploid or half number of the
autosomes.) So:-
77
normal
disjunction
of the sex
chromosomes
1\
(9
0 0
eo
(9
1\
nondIsjunction
of the sex
chromosomes
Fig. 23.
This failure on the part of the X-chromosomes
to disjoin is known as non-disjunction of the
sex chromosomes. Now, assume that the sperm
with two X-chromosomes fertilises a Y-chromosome-bearing egg. An XXY individual will
result and it will be a male. But suppose this
was a cinnamon cock. (It will be desirable, in
this particular discussion, to use a different
method of symbolisation.) This cinnamon cock
would have a sex chromosome constitution that
can be represented as (cnX) (cnX)Y, the brackets
include an X-chromosome and the genes it
carries. Then the mating cinnamon cock X
wildtype (non-cinnamon) hen becomes:(cnX) (cnX) Y X (+X) Y
But this cock, because he possesses three sex
chromosomes instead of two, will produce four
kinds of gametes instead of one. Into a ripe
spermatozoon there can pass normally one.
X-chromosome for the reason that the two X's
in the immature sperm cell disjoin and each X
passes into a separate sperm. But in this case, if
into one sperm, passes one X-chromosome, into
the other there will pass an X together with a Y.
Or, if into one sperm there pass two X's, then
78
into another will pass the Y-chromosome alone.
So four kinds of sperm result, so:(cnX); (cnX) Y; (cnX) (cnX); Y
and these will be available for the fertilisation
of two kinds of eggs, those including the Xchromosome and those with the Y. Fertilisation
will yield the following types:1. Egg (+X)X (cnX) sperm = (cnX) (+X) w!ldtype cock
2. Egg (+X)X (cnX) Y sperm=(cnX) (+X) Y wlldtype cock.
3. Egg (+X)X(cnX) (cnX) sperm = (cnX) (cnX) (+X) wlldtype
cock.
4 Egg (+X)XY sperm = (+X) Y wlldtype hen.
5. Egg YX (cnX) sperm = (cnX) Y cinnamon hen.
6. Egg YX (cnX) Y sperm = (cnX) YY Clllnamon hen
7. Egg YX (cnX) (cnX) sperm = (cnX) (cnX) Y cmnamon cock.
S Egg YXY sperrn=? does not appear.
No. I is a normal heterozygous non-cinnamon
cock; No. 2 is a normal cock so far as his colour
is concerned, but he, like his father, is nondisjunctional and will, in the breeding pen, give
unexpected results; No. 3 is a normal cock to
look at, but like No. 2 is abnormal in respect of
the number of his sex chromosomes. No. 4 is
a normal hen, normal in every way but, because
of her plumage colour, certainly unexpected.
She has disobeyed the rules of criss-cross inheritance, for she has the character of her mother.
This is due to the fact that she derived her
Y-chromosome from her father and not as the
normal female should from her mother. Her
father was abnormal in possessing a Y -chromosome. She received her X -chromosome from
her mother, whereas, normally, the daughter
79
receives this chromosome from her father. It is
because she has got her mother's X-chromosome
that she is like her mother, not cinnamon. NO.5 is
a perfectly normal female. No.6, though she has
two Y-chromosomes, is a female of the expected
colour. No. 7 is an exceptional cock. He is
exceptional in being cinnamon when he ought
not to be, because he received two X -chromosomes [rom his father by way of the sperm
whilst from his mother he received a Y -chromosome. He is like his father because he has his
father's X-chromosomes, and because he did
not receive, as he should have done, from his
mother, an X carrying the wildtype allele of
cinnamon.
Non-disjunction of the sex chromosomes will
sooner or later be encountered if the mating
cinnamon cock X non-cinnamon hen is made at
all extensively. It has nothing to do with the
halfsider; it is discussed at this point because it
is an example of another kind of fault in the
mechanism of the distribution of the chromosomes.
But to return to the autosomal colour mosaic,
the halfsider, that has been reported fairly
frequently; there are at least a score described
in budgerigar literature. It is worthy of note
that in every case it is the blue chromosome that
has been eliminated, and that in the majority it is
the right side of the body on which the recessive
colour is revealed. The explanation of a series
of these recently reported in The Budgerigar
Bulletin is as follows:-
80
-
1.
Phaenotypes
and genotypes
of parents
Father
Mother
light
green
+
light
green
+
b
b
--
2.
cobalt
y b D
- -+ b+
3. greywmg
cobalt
gr b D
-y b+
Dark
4.
green
b D
Phaenotype
and genotype
of blcolour
Rt. side
Lt. Bide
Sex
ltght
green
+
blue
- -
--
-b
b
white
cobalt
Y b D
y b +
white
cobalt
Y b D
y b+
white
blue
Y ---
whIte
cobalt
Y b D
--
-- - - - -
--
Y
b +
Y b +
greywing greywing
cobalt
blue
gr b D gr
y b+ y b +
Dark
cobalt
cobalt
green
b D
--++
----D
b D
b
b+
++
Dark green
Dark
ltght
ltght
5.
green
yellow
Type I
green
y b+
+ +D y - - - Y +D
-- - - - - -y ++ + b + + + + + ++
6. skyblue Dark green skyblue Dark green
--+D
+D
b+
--
--
----
--
--
--
--
--
b +
b+
b+
b+
- - -
a
a
~
a
Explanation
EhmmatlOn of autosome beanng wudtype allele of the
blue gene at first
divisIOn of fertilised
egg.
EltmmatlOn of autosome bearmg the
genes band D at
fust diVISion of fertiltsed egg
do.
Elimill~tion of autosome b~anng normal
alleles of band D at
fIrst divIsIOn of fertlhsed egg.
EltmmatlOn of autosome bE'aring D and
~ the normal allele of
blue.
a
do
= loss of chromosome
You will have noted that the yellow
chromosome precedes the blue chromosome.
This is merely convention. We agreed to call
the yellow Chromosome II, simply because the
yellow variety was known before the blue. If,
in addition to these two chromosomes, the
X -chromosome entered into a formula, it would
come first.
THE BI-CO~OURED OR HALF SIDER
81
It is not improbable that the pied budgerigars
which have been described in the literature are
birds in which Chromosome II-the yellow
chromosome-has been eliminated, in whole or
in part, during the later stages of development.
Such specimens should be examined in the light
of this suggestion. However, if pieds breed true,
as has been reported, this explanation will not
hold.
VII
NEW MUTANT CHARACTERS
WING to the fact that a new mutant form,
when first it appears, is usually not studied
as a problem in genetics, the available
information concerning such new varieties as the
fallow, lutino, or the albino is unnecessarily
confusing and complicated.
A considerable
number of specimens of these new varieties are
in existence, and a few breeding records have
been made available. But the reports of different
breeders are so very various, even mu~ually
contradictory, that for the present it is u::eless
to attempt to discuss the genetic basis of these
new mutant characters.
From what has bem written about albino and
lutino, however, it seems certain that the albino
character, like the ordinary white, is not the expression of a single gene mutation but is due to
the interaction of two genes. These, in the case
of albino, are lutino, a gene that yields a pure
yellow plumage and pink eyes, together with the
blue gene with which we have becotne so well
acquainted. Thus, the genetic fortnulre for the
two new characters are
O
Iu
Iu
Iu b
=
-
Iutmo,
-
Iu b
=
albmo
For the present it is impossible to state on which
chromosome the lutino is situated, though it is
suggested that its place is upon the X.
82
What is known of the buttercup, however,
lends support to the view that it has been
obtained "by selection" out of the old yellow.
As this is precisely the sort of problem which
an understanding of genetical methods brings
easily within the scope of rapid solution, we shall
digress for a moment and discuss the general
question of selection.
It is quite possible that such a type as the
buttercup may have been produced by selecting
birds with less and less suffusion in each generation and mating them together. It is possible,
but there is no guarantee that such a result can
always be produced by such a method. Selection
in a given direction succeeds in some cases and
fails in others. The reason is perfectly simple.
We realise that while we know something about
some half-dozen of the genes of the budgerigar,
there must be thousands of others of which we
know nothing. Among this enormous number
of unknown genes it is eminently possible that
there may be a few which, in a greater or less
degree, affect plumage colour in the same
direction as the yellow gene. The effect may be
too slight to be noticed in the green types; their
action may be recessive, they may possess
wildtype alleles which are more common; in fact,
there may be many reasons why the action 9f
such genes is not as obvious as that of the
yellow gene itself. But ne~ertheless, it might
happen that chance would bring together some
of these modifying genes (as they are called)
into some strain of yellows; their effect would
be to lessen the amount of visible green suffusion.
84
Consequently, the breeder who wishes to select
would seize upon such with joy. By in-breeding
he would keep these modifying genes together,
and also collect any more that happened to be
in the strain: in this way in time he would
obtain a type that was the effect of the old yellow
gene plus two, three, four, or even more pairs of
recessive modifiers. But when he had collected
all the existing modifiers he could go no further.
By carefully continuing to in-breed the type be
had so selected, he could, however, always be
sure of perpetuating it. What be must not do is
to outcross to any other strain, even a yellow
strain, because then all the modifying genes
would become once more dispersed, ~nd the
selected type would in consequence revert at
once to the old type.
There, are cases, as we have said, in which
selection is not effective, because there are no
modifying genes to ~lect, though a single gene
may vary so considerably in its expression as to
suggest the presence of modifying factors.
Is it possible to prove conclusively that such
a type as the buttercup is really the result of
selection of heritable modifying genes, and not
of a new mutation of the old yellow gene?
This question could be solved by appropriate
breeding tests covering two generatiom. The
first cross would consist of the mating buttercup
of the purest variety to a yellow with as much
green suffusion as possible. The result of this
mating would consist of all green suffused
yellows, and the effect would be the same whether
we were dealing with a number of recessive
modifiers of yellow in addition to the yellow
gene itself, or simply with two different alleles
of yellow. In the former case the recessive
modifiers, being present only in the heterozygous
condition, would be ineffectual, and so the yellow
gene alone would manifest its presence. In the
latter case there would be two alleles of yellow
present, and, as usually happens in such cases,
there would be a dominance of that allele which
showed the less pronounced departure from the
wildtype, in this case the suffused type of yellow.
The two possibilities may be diagrammatically
represented as follows. The symbols m!, m2,
m3 represent modifying genes, yl the old yellow
gene, and y2 a hypothetical new yellow allele
producing pure yellow plumage.
1st possibility: the presence of modifying genes
in addition to yl:_
yl
ml
buttercup
m3
m"
yl
ml
3
m
m"
yellow
yl
X
yl
+
+
yl
ml
m"
m3
yl
+
+
+
+
+
+
+
PI
FI
2nd possibility: the presence of a new yellow,
y2;_
buttercup
y'
yellow
yl
x
y'
yl
SO, from the FI generation nothing conclusive
could be extracted. We should have, in any case,
G
86
ordinary yellow young with green suffusion .
. But the next cross would decide the question,
and if we raised a sufficient number of
youngsters this would tell us not only whether
modifiers were present or not, but approximately how many modifiers (if any) were concerned.
We mate, then, the FI yellow birds back to
pure buttercups, and note the types of plumage
that appear in the back-cross generation.
If we are dealing with modifiers, the plumage
colours will range from yellow with green
suffusion through several intermediate sh<ldes to
pure yellow. But if we are dealing simply with
two different yellow alleles, the plumage lf the
back-cross generation will consist of 50 per cent.
suffused, and 50 per cent. pure yellow.
If three modifiers are concerned, the backcross will be represented as follows:x
y
rn 1
rn'
rna
y
+
+
+
and the offspring will consist of eight different
types, among which one will be the pure buttercup yellow, which will have inherited from both
parents the three recessive modifiers; one will
be a deeply suffused yellow having inherited only
the normal allelomorphs of the three modifiers
from the heterozygous parent; there will be three
who have inherited two of the modifiers in the
homozygous condition; and three who have
inherited only one. These six will possibly show
perceptible differences one from another, but as.
NEW MOT ANT CHARACTERS
we know nothing of their nature, this cannot be
expected with certainty. The ratio of I clear
yellow in every 8 will nevertheless indicate that
clear yellow requires the presence of three
recessive modifiers in homozygous condition.
If, instead of three modifiers, four were
concerned, the ratio would be one clear yellow
in every sixteen.
If buttercup yellow were the result of mutation
of the yellow gene, however, the back-cross
would be represented thus:yl
y'
y2
y'
X
y'
y2
yl
y2
50 per cent.
50 per cent.
There would be the simple assortment of two
allelomorphs with each other giving only two
different types.
VIII
METHODS FOR THE GENETICAL
ANALYSIS OF A NEW MUTANT
CHARACTER
NE principle must guide the breeder in his
analysis of a new character, and that is that
he must always test the unknown against
the known. To mate up a bird with a new and
therefore unknown character to another whose
genetic constitution is unknown is the shortest
route to confusion. Ideally, two stocks should
O
be used, a pure light green
!!!!
(that is,
a light green not carrying cinnamon, yellow, or
any of its alleles, blue or Dark), and a cinnamon
white cobatt
cnybD
cnybD)
- - - - cocks and - - _ - hens
( cnyb+
. yb+
With individuals of these stocks the new mutant
should be mated.
Sometimes a new mutation is of a kind that
indicates fairly obviously that it is a new variety
of the yellow gene or of the blue gene. For
example, among the offspring of a matillg in
which yellows are to be expected, in addition to
the yellows, or in place of them, a new colour,
seeming to the eye to be related to yellow,
appears. At once one would act on the assumption
that a change had occurred in the yellow gene
88
~ENETICAL
89
ANALYSIS OF A NEW MUTANT CHARACTER
and that the new mutant would probably prove
to be an allele of yellow. So the new mutant
should be mated with a yellow in order to get
the intermediate colour, the blending effect.
These intermediates should then be put back to
yellows, when two classes, yellows and intermediates, should appear among the offspring.
If what seemed to be a new shade of blue appeared,
then it should be mated to blue to get intermediates, and these should then be put back to
blues.
Again, if the new mutant character appeared
in a single hen, the assumption would be that a
new sex-linked recessive mutation had occurred.
The hen should be mated up with a light green
cock, and then next year a son of this mating
should be put back to his mother. At the same
time sons and daughters should be mated inter se
in order to get in the next generation more hens
with the new character. Let a=the new sexlinked recessive mutant gene, so:-
+
+
+
-
a
X
X
-
PI
+
FI
a
+
+
+
wtldtype cock
wlldtype cock
/
F2
+
a
wlldtype hen
hen':'wlth new
a
(carrIer)
Fl
son
./
character
back-cross
+
a
a
X
-
mother
90
+
a
+
a
a
wildtype cock
(carrier)
a
cock Wlth new
character
wild type hen
hen with new
character
But, if there is no obvious suggestion that the
new character is an allele of yellow or blue, and
if it does not seem to be sex-linked because it
has not appeared in a female, then it is best to
adopt a straightforward routine method of
analysis. Examples of the so{t of character that
is meant are the beak deformity or the French
moult which, according to many breeders,
would seem to be genetic in their origin; chat is
to say which because of their orderly distribution
among the generations, and because they ,:annot
be at all easily referred to any faults of husbandry
and nutrition, seem to be hereditary characters.
The questions to be asked and answered are:I.-Is the new mutant character a dominant
or a recessive?
2.-1s it sex-linked?
3.-Is it autosomal?
4.-18 it linked with yellow?
5.-Is it linked with blue?
6.-Is its gene on a chromosome other than
I, II, and III?
I.-Is it a recessive or dominant?
If, when mated to a wildtype (homozygous
light green), none of the F1 individuals exhibit
it, the new character is presumably a recessive,
and in the F2 it will be exhibited by about 2. 5
GENETICAL ANALYSIS OF A NEW MUTANT CHARACTER
91
per cent. of the individuals, both males and
females. If half of the PI individuals exhibit the
new character, then it may be dominant, the
original individual being heterozygous. If this
is the case,. then these PI individuals with the
new character mated back to wildtype will give
a 50: 50 ratio, half exhibiting the character, the
other half not doing so.
RECESSIVE
++
X
+a
X
aa
Pi
+a
Fi
I
I
aa
1
25 per cent.
with new
character
+a
2
++
1
'-.r---J
75 per cent
wlidtype
Fa
DOMINANT
++
back-cross
~+
A+
50 per cent.
with new
character
A+
X
I
++
PI
Fl
++
50 per cent.
wlidtype
2.-Is it sex-linked?
If the new character appears in a single hen,
the probability is that it is a sex-linked recessive.
If so, the hen, when mated to a light green cock,
will produce all light green offspring. Back-
92
THE GENETICS OF THE BUDGERIGAR--
crossed to her son, she will then produce equal
numbers of sons and daughters showing and not
showing the new character. But if the FI sons
and daughters are mated together, only female
offspring will show the character. The new
character may be a sex-linked dominant, however,
in which case it would be just as likely to appear
in a cock as in a hen. If it happened to be a cock,
sex-linkage could not be proved in the first
generation. But if one of his daughters (showing
the new dominant character) be crossed to a llght
green, then, if the character is really sex-linked,
only her sons will show it.
3.-If it is not sex-linked, then it is autosomal.
4.-Is it linked with yellow or with bL e, or is
its gene on a chromosome other than
I, II, and III?
Tests for linkage are usually somewhat complicated, but chance may simplify the proceedings
considerably. The reader, of course, realises that
the object of such tests is to discover whether
the new gene is located on the same chromosome
pair as yellow (Chromosome II) or on the same
chromosome pair as blue (Chromosome III). If
it is on Chromosome II it will give free recombination with blue but not witb yellow;
while if it is on Chromosome III it will give free
recombination with yellow but not with blue.
Now, if the new mutation occurred in a stock
which already carried both yellow and blue
(tbat is, a white bird) the rest is comparatively
simple.
The white bird showing the new
character (which we shall call a) should be mated
9}
to a light green. We may represent this mating
symbolically thus:II
Y (a')
III
b (a?)
IV
(a?)
y (a?)
b (a')
(a?)
x
II
III
IV
+
+
+
+
+
+
As we do not know where a is located, we map
out temporarily the three positions which are possible, namely on either of the two chromosomes
which are already marked by known genes, and on
an unknown chromosome which we call Chromosome IV. The genetic constitution of the FI offspring of this mating may be symbolised thus:y (a?)
b (a?)
(al)
+ +
+ (+)
+
They are wildtype greens heterozygous for
yellow, blue, and the new character. Some of
these triple heterozygotes we mate together.
Others we must back-cross to the mutant
parent. This latter mating we symbolise thus:y (a?)
b (a?)
(a?)
y (a;»
b (a?)
(a?)
+
+
(+)
y (a')
b (a?)
(a?)
-----x----(+)
(+)
We shall co~sider the results of this mating,
for this will decide the question of the real
position of a once for all when sufficient numbers
are obtained. If a is on the second chromosome,
it will show linkage with yellow; thus:y
ya
b
a
b
++
ya b
whIte + the new
chmacter 25%
+
ya
x
+
-'-
ya b
yellow + the
new character
25%
ya
b
ya
b
+ +
y
b
a
b
blue
25%
I
+ + +
y a
green
25%
b
94
There will be four kinds of offspring in equal
numbers: white, yellow, blue, and green, and in
50 per cent. of these the new character will
appear. But it will at once be remarkable that
all those that show the new character are either
yellow or white. We know therefore that the
mutation has occurred in a gene which is situated
on Chromosome II rather close to yellow.
However, it may be on the same chromosome as
yellow without being very close to it, and in
that case if a sufficiently large number of offspring
are raised, exceptional individuals will appear,
the result of crossing over between yellow and
the new gene in the heterozygous parent; these
will be blue or green birds exhibiting the new
character. Thus, by crossing over ~ : becomes
y
+ and so instead of yellow and a being 011 the
+a
same chromosome and both normal alleles on
the other, each chromosome now carries only
one of these recessives with the normal allele
of the other. When this cross-over sperm or
egg which carries the gene a fuses with a sperm
or egg of the homozygous parent (all of which
carry both y and a) the result is an individual
which exhibits only a, yellow being in the
heterozygous condition. The blue gene mayor
may not also be present in homozygous condition.
If it is, the bird is also blue, if not, it is green.
But if the position of a is on Chromosome III
near blue, then of course the new character will
show linkage with blue in this test. So:-
b
Y
y
b a
a
Y b a
whlte wlth the
new character
25%
b a
y
+ + +
+
95
X
ba
y
b a
y
b a
y
blue wlth the
new character
25%
-
+ ++
+ +
--
y
b a
y
yellow
25%
b a
green
25%
As before, the birds will be white, blue,
yellow, and green, in equal numbers, but now
the new character will appear in the blues and
whites and not in the yellows or greens. But
again exceptions may appear due to crossing
over. These exceptions, however, can always
be recognised as exceptions if the numbers
obtained are large enough to show that they
are in a perceptible minority.
A last possibility remains to be considered.
The new gene a may be neither on Chromosome
II nor on Chromosome III. We must represent
the back-cross therefore thus:II
Y
III
b
IV
II
III
a
y
b
IV
a
+
+
+
y
b
a
X
Equal numbers of whites, greens, yellows, and
blues all are obtained, and the new character
will be evenly distributed among the four types,
thus:y
b
a
white
white wlth the
new character
y
b
a
Y
+ +
y
b
a
y
+
a
yellow
y
b
+
y
b
a
y
b
a
yellow with the
new character
I
96
y
b
y
a
blue
b
a
green
blue wIth the
new character
+
b
+
y
b
a
+
b
a
+ + +
green with the
new character
b
a
+ +
a
y
Thus, if a new mutant occurred in a bird
carrying both blue and yellow, its position with
regard to all other known genes could be
ascertained in a couple of generations. Moreover,
not only the fact of linkage (if this existed)
would be established, but the closeness or
otherwise of linkage would be indicated by the
proportion of "exceptions" which occurred.
If the new mutation occurred in an individual
which carried only blue or yellow the same
plan could be followed-the outcross to wildtype,
and the back-cross to the mutant parent-but
this would give knowledge of linkage with respect
to one pair of chromosomes. only. There is,
however, another type of linkage test which
could be usefully employed in such cases, and
also in cases where the mutation had occurred
in an individual which carried no other known
genes; that is, a wildtype light green. It will
be necessary to consider only this last case. The
new mutant is first mated to a white bird. We
shall assume, to begin with, that the new gene
is neither on Chromosome II nor on Chromosome
III. The mating is therefore represented thus:II
III
IV
II
+
+
+
+
a
y
III
b
y
b
a
x
IV
+
+
97
The constitution of the FI then would be:II
Y
III
b
IV
a
+
+
+
and they would all be wildtype in appearance.
The next step would be to mate these together
in pairs, brother by sister, and raise as large an
F2 as possible. Since the three genes are each
on a different chromosome they will assort
independently of each other in the gametes of
both the parents, and the offspring will exhibit
every possible recombination of the three
characters. It will be found most convenient
to consider the characters in pairs, taking a first
with yellow, then a again with blue. There is
no need to consider blue with yellow, for we
know already that these will appear in the
proportion 9 wildtype, 3 yellow, 3 blue, and
I yellow blue (white).
But, disregarding blue, we should also get:9 neither yellow nor the 1).ew character, 3 yellow,
3 the new character, I both yellow and the new
character; and, disregarding yellow, we should
get:9 neither blue nor the new character, 3 blue,
3 the new character, I both blue and the new
character.
We consider the characters thus in twos
purely for the sake of convenience. If we map
out on a checkerboard all the possible types of
gametes in the parents and of the union of these
98
in the offspring, we should get the following
proportions in every sixty-four:27 wildtype
3 yellow blue
3 yellow new
3 blue new
I yellow blue new
9 yellow
9 blue
9 new
We should need very large numbers to be able
to observe these proportions, hence the breeder
will usually find it more practicable and entirely
sufficient to consider two pairs at a time in order
to know whether recombination occurs freely
between the three.
Now let us suppose that a is not on a new
chromosome but on the old Chromosome II.
The cross to white would be represented thus:III
II
II
III
b
y+
+
X
b
y+
a
+
and the Fl individuals would be of this constitution:-
+
+
a
II
III
II
III
+ a
y+
+
+ a
y+
+
b
X
b
Now, mating these together we should obviously
not get independent recombination, for both a
and yare on the same pair of chromosomes.
Moreover, a is on one member of the pair and
y is on the other member. And since each parent
can contribute only one member of that pair
to the offspring, the latter may receive either:-
99
(1) one
a from each parent, in which case it
would show the character associated
with a,
(2) one y from each, in which case it would be
yellow, or
(3) an a from one parent, and a y from the other,
in which case it would be wildtype,
heterozygous for both a and y.
But there would be no offspring showing both
yellow and the new charac~er.
This would be the proof that the new gene
was located on Chromosome II. And although
it is not impossible that exceptions should occur,
yet they would be extremely rare in a mating
such as this where both parents carried the two
recessive genes, a and y on different members of
the chromosome pair. In order that exceptions
be produced, it would be necessary, first, for
crossing over to take place in the germ cells of
both parents, and then for the gametes carrying
the recombined recessives to meet in the process
of fertilisation. Now it is known that in many
species crossing over is much more rare in one
sex than in the other-in some species, indeed,
it is entirely absent in one sex-and this is
possibly so in the budgerigar as well. But since
also crossing over produces by reconstruction
two new arrangements of the' genes, thus: a y
and + + (instead of a + and + y) there would
only be a very slender chance of the a y meeting
a similar a y when the great majority of the
germ cells carried the a + and the + y type,
and since any rare cross-overs that happened to
100
++
be present might equally well be the
as
the a y type. We should therefore expect that
the existence of linkage would be manifested by
the complete absence of recombination of the
new character with yellow. It is unnecessary to
repeat the process to illustrate the existence of
linkage with blue. If the new gene happened
to be on Chromosome III, there would be no
recombination with blue in the F2 generation.
Having established the position in the chromo:..
some complex of the new gene, the breeder has
now the advantage of knowing the exact
possibilities of future work with the new variety.
He will know, for instance, what combinations
he can first hope to manufacture. These will be
combinations with non-linked genes. He will
know that since combinations of linked genes
depend on rare crossing over and chance fertilisation of suitable gametes, he cannot expect to
obtain these until he has closely in-bred his
heterozygotes for several generations. He will
know when outcrossing may be practised without
danger of confusing his results. In short, he will
know the exact possibilities of his material, and
will be able to use this knowledge always to the
best advantage, and will not waste time 'attempting the impossible.
IX
IN-BREEDING
N-BREEDING is the system of breeding in
which closely related individuals are mated
for the production of offspring. Out-breeding
is that system in which unrelated birds are mated.
But since all budgerigars are related more or less
closely, the difference between the two systems
is one of degree. In-breeding, in practice, comes
to mean the mating of father to daughter, mother
to son, brother to sister, uncle to niece, nephew
to aunt, first cousin to first cousin. Line-breeding,
which is in-breeding in a milder form, implies
the mating of two individuals belonging to the
same family, to the same pedigree, both individuals having a number of ancestors in common.
Out-breeding involves the mating of individuals
belonging to different strains or aviaries.
I
In-breeding is a technique used universally and
successfully for the definite purpose of purifying,
in the genetical sense, a stock, and for the
fixation and augmentation of those hereditary
qualities which are regarded as desirable by
the breeder. In-breeding is almost invariably
attended, particularly in the earlier stages of its
application, by undesirable effects, e.g. diminution
in size, decrease in fertility, the app<;ar~ce of
abnormal forms, slackened growth rate on the
part of the offspring. Often, indeed, its continuance has resulted in calamity, e.g. complete
sterility, inability to rear the young, profound
101
H
102.
general weakness ending in death.
So that
in-breeding can yield results that are distinctly
advantageous, and also results that are definitely
disadvantageous.
Out-breeding is the technique universally and
successfully used for the definite purpose of
raising the level of general vigour in a failing
stock, of introducing into a stock some property
which it does not itself possess, but which is
exhibited by another stock or strain, and especially
for the purpose of obtaining the so-called hybrid
vigour. This hybrid vigour, as is well known to
breeders of commercial stock (e.g. mules, crossbred cattle, sheep, and pigs) is the peculiar
property of the FI generation, and it is recognised
that though the FI is fairly uniform, in succeeding
generations this uniformity gives place to a
veritable epidemic of variation, and the hybrid
vigour becomes so dispersed as to be lost. The
FI individuals are produced because their commercial qualities are so excellent, but they are
not used for further breeding.
If the breeder wishes to add to his own stock
some quality possessed by some stock other
than his own, he attempts to introduce this by
purchasing one or more individuals and mating
them up with his own. But in so doing he is
introducing not only this particular hereditary
character, but dozens of others of which he can
know nothing. He will require to purge his own
stock as time passes of all the qualities brought
thereinto which he does not want, preserving
only that quality he wishes to secure. Outcrossing is always followed by the exhibition of
IN-BREEDING
a lack of uniformity, and in practice is always
succeeded by a prolonged phase of in-breeding.
So that out-breeding is followed by increased
general vigour but also by disturbing variation.
It now becomes clear that in-breeding and outbreeding are not different methods of achieving
the same end: they are different tools used for
different purposes, and both will be employed
to meet different situations at different times by
every breeder of intelligence.
If the breeder has in his stock a desirable
quality (some colour shade, feather length, head
shape, for example, which is hereditary) that is
variable in the degree of its e~pression, and if
he wishes to stamp uniformity in respect of this
quality upon his stock, then he must in-breed.
But in so doing he must at all times exercise
the greatest care in the choice of his birds for
mating. It is not enough to pay attention only
to the quality he seeks to improve: he must also
and ceaselessly ensure that every bird he uses for
breeding shall be in every way desirable. Weakly
birds, birds with poor records, or with relatives
with poor records, must not be used, no matter
how excellent they are in respect of the particular
character that mainly interests the breeder.
There is nothing wrong with in-breeding
itself; it is good or bad according to whether the
birds exposed to it are good or bad. _, If in the
individuals there are many recessive genes
(necessarily in the single dose) corresponding to
characters that are deleterious or undesired, then
104
in-breeding will be expensive, for it will expose
these characters. It is dangerous for the sole
reason that it is far more likely that two closely
related individuals will possess the same recessive
genes than that two unrelated indiv iduals should
do so. Therefore it follows that in-breeding is
more likely to involve the meeting of two
individuals, each heterozygous for the same
undesirable genes than is out-breeding, and when
two such heterozygous individuals meet, the
recessives appear.
In-breeding uncovers the
hidden, whereas out-breeding tends to keep the
undesirable unexpressed by preserving the heterozygous state.
But obviously, in-breeding is only harmful if
the hidden is undesirable; if the recessives are
advantageous or not disadvantageous, then their
exposure is to be welco~ed. In-breeding exposes
the good as well as the bad, and the actual result
is determined by the genetic constitutions of the
individuals concerned. Furthermore, if the hidden
is undesirable, and yet is exposed, it can then be
discarded. The records will show which individuals carry the unwanted recessive genes and
which do not, and so, by appropriate selection,
the stock can be purged. Of course, purgation
will be most expensive if the stock happens to
be full of undesirable recessives; on the other
hand, if there are but few, then a course of
in-breeding cannot be otherwise than beneficial,
taking the long view, for, along with the undesirable there will be the exposure of the
advantageous and the profit will outweigh the
loss.
IN-BREEDING
10 5
If the breeder decides to expose his stock to a
course of in-breeding, the best method to adopt
is to divide two or three broods out of the same
parents into pairs, six pairs let us say, and to
use these as the foundation of six different lines
which should be kept quite distinct. In each
generation brother should be mated to sister,
but always the birds selected for mating should
be excellent specimens. It is not improbable
that, after the third or fourth generation, one or
more of these lines will have come to an end,
through sterility or difficulty in rearing. But at
the same time it is to be expected that in the case
of other lines the individuals will not only be
more uniform among themselves, but will show
a definite advance upon the foundation stock.
The different lines will soon come to differ one
from the other in respect of the degree of
expression of the various characters, and among
them there most probably will be one that
delights the eye of the breeder.
The reason for this dissimilarity between the
different lines is that the original parents were
heterozygous for a great number of genes. All
birds that have not been in-bred are. Segregation
and gene distribution during the formation of
their gametes would ensure that their offspring
which formed the foundation stock of the six
lines would come to differ among themselves.
Of course, they would be all alike in respect of
those genes for which their parents were homozygous, but if these were homozygous for a
score, they would be heterozygous for hundreds
probably. Some of the offspring would receive
l06
the dominant gene of a pair, others the recessive,
and since there were many pairs a considerable
number of combinations and permutations would
be possible.
The subsequent in-breeding of these six lines
would then lead to the exposure of different
recessives. The more harmful, being expressed,
would soon lead to the extinction of the line.
But such lines as had, by chance, received the
advantageous recessive genes would flourish.
I t is as though one took a heap of good and bad
genes, mixed them, and then from the heap took
several handfuls. One could get mostly bad or
mostly good. In this way the hidden is revealed
and the stock can be purged.
But it is to be emphasised that during the
course of in-breeding the greatest watchfulness
must be exercised at every stage; every weakling
must be discarded, and careful attention given to
growth rate and fertility. It is useless to in-breed
on a small scale or timidly. If only a few birds
are involved in the process the time will soon
come, in all probability, when the breeder will
be faced with impending disaster; he will then
have to choose between dropping the whole
thing, or else of introducing fresh blood, and
neither of these is in-breeding. The secret of
successful in-breeding is to start on a sufficiently
large foundation and select thereafter with the
utmost rigour.
If at the end of such a course of in-breeding
the breeder should find himself in the possession
of two or three lines, then it will be possible for
IN-BREEDING
10
7
him at any time to make crosses between any
two of them, and he can do this whenever he
It is most desirable, even
senses danger.
essential, to have two or more such related lines
which, though they differ one from the other,
do not differ too much, for it is always necessary,
sooner or later, to infuse fresh blood into any
strain and to make a wide out-cross in such a
case would be sheer lunacy, for all the benefits
of in-breeding would be flung away. This would
be out-breeding with a vengeance, the offspring
would be heterozygous for a multitude of genes,
and among their progeny there would be an
appalling lack of uniformity.
So we may conclude that there is nothing good
or bad in in-breeding itself; there is often a great
deal of good and a great deal of bad in the
individuals exposed to it; if in them there is
hidden wretchedness, in-breeding WIll reveal it;
if there is hidden excellence, this also will be
revealed. It has already been remarked that
among the hidden things to be revealed there
may be recessive mutant genes that correspond
to new varietal colours. The breeder who seeks
to increase his 'chances of creating a new variety
would be advised to in-breed yellows systematically, for the reason that yellow would seem
to be the most mutable of all the genes so far
known.
Out-breeding gives results that ~gain are
determined by the genetic constitutions of the
birds concerned. It yields heterozygous offspring
and so, if the genetic constitution of the two
108
parents are such that the good of one reinforces
the good of the other, or cancels out the bad of
the other, the results are excellent. The essential
feature of out-breeding is the pooling of genes
derived from two different sources; if the two
sets are complementary or compensatory, all is
well, but if undesirable merely reinforces the
undesirable, then the results are most unsatisfactory. The results cannot be predicted
since in out-breeding as in in-breeding we are
dealing with the unknown.
X TERMINOLOGY
A
WORD must be said in conclusion about
the descriptive terms popularly employed
by budgerigar breeders. While it would be
a difficult matter to effect a complete reform of
the terminology in current use, and not altogether
desirable to do so, it is absolutely necessary in
our opinion that the breeder should be able to
transform popular into strictly genetical terms,
as well as to express in popular language any
genetic formula. As the foregoing pages will
have sufficiently demonstrated the principles on
which genetic terminology and formula: are
based, it will be necessary only to summarise in
tabular form the different ways of describing the
known varieties. In this table we shall mention
only those types which carry no recessive genes
in the heterozygous condition; that is, those in
which the genotype can be inferred from the
phaenotype.
The name cinnamon is suggested instead of
cinnamonwing as being less cumbersome.
The expression "light cinnamon" is given as
the popular name for "cinnamon greywing"
following the suggestion proposed in a recent
issue of The Budgerigar Blllletin, but we strongly
disapprove of it even as a popular/term for it is
unnecessarily ambiguous and misleading. In
this case, the plain genetic description is so simple
that there is no reason why it should not be used
as a popular name.
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114
The use of the word "split" or the split
sign / to designate the presence of recessive
factors in the heterozygous condition is universal,
and there is no reason why it should not remain
so, provided the meaning is clearly defined and
not in contradiction to genetic principles and
usage. The following definition proposed by
Mr. F. S. Elliott is an excellent one, and since it
makes the word synonymous with the more
cumbersome "heterozygous," its logical application to the appropriate genotypes will be found
to be both convenient and in keeping with
genetic principles.
"When we say that a bird' is 'split' a certain
colour we mean that it itself po.§sesses the hidden
power to produce, when suitably mated, a certain
percentage of young of that colour over a fair
number of matings. The bird itself possesses
the colour factor or factors corresponding to the
split colour, and (except in the case of sex-linked
characters) it is necessary for the bird to which it
is mated to possess the same hidden factor or
factors also."
According to this definition, therefore, all the
characters that are apparent in the bird are
written on the left side of the split sign, and only
those recessive characters which are present in
the heterozygous condition are written on the
right. This rule, in fact, is generally adhered to:
it is probably only in the case of white that
confusion arises. It must be remembered that
"white" is the result of two different recessive
factors, namely yellow and blue, which must both
be present in the homozygous condition. For a
TERMINOLOGY
bird, therefore, to be "split white," it must carry
both yellow and blue in the heterozygous
condition. Thus the expression "light green/
white" means that a light green bird carries both
yellow and blue in heterozygous condition, and,
if mated to blue, will give 50 per cent. blue; if
mated to yellow will give 50 per cent. yellow;
and if mated to white will give 25 per cent.
white offspring. Logically, also, one may use
the expression "cinnamon split white" or "greywing/white," but not "blue/white" or "yellow/
white." The reason is obvious. A yellow bird
must carry yellow in the homozygous con<;lition,
and a blue bird must carry blue in the homozygous
condition; to place blue or yellow, therefore, on
one side of the split, and white on the other,
w<:mld indicate that the bird possessed one of
these factors in triplicate, which would be a
gross genetic abnormality.
The correct expressions are, of course, "blue/yellow" and
"yellow/blue."
XI GLOSSARY.
ALLELE, ALLELOMORPH (al-Ieel, al-Ieel-o-morf):
one of a series (two or more) of genes which
occupy one and the same locus in a particular
chromosome. Also one of a series of hereditary
characters whose genes occupy one and the
same locus in a particular chromosome.
AUTOSOME: a chromosome other than a sex
chromosome.
BACK-CROSS: the mating of an PI individual with
one of its parental types; also applied to the
progeny of such a mating.
CHROMOSOME (kro-mo-zome): one of the bodies
into which the chromatin material of the
nucleus of the cell is arranged, visible as
separate objects when the cell is dividing.
CROSSING-OVER: the interchange of material
between the members of a pair of homologous
chromosomes.
CYTOPLASM (si-toe-plazm): the protoplasm (living
stuff) of the cell excluding the nucleus.
DIHYBRID: the Pl produced by parents-differing
one from the other in respect of two distinct
pairs of allelomorphic characters.
DOMINANCE: the ability of a hereditary character
to be e:x:ptessed when the gene corresponding
to it is present only in the heterozygous state,
and is associated with the gene for the alternative recessive character.
II6
GLOSSARY
117
a gene is said to be dominant when
the character corresponding to it is expressed,
though the gene is present only in the simplex
state and is associated with its recessive allele.
DOMINANT:
Fl (ef wun): an individual or generation produced
by the mating of two individuals differing one
from the other in respect of the members of
one or more pairs of allelomorphic characters.
F z (ef too): an individual or generation produced
by two individuals belonging to an Fl'
the union of egg and sperm,
essentially the fusion of their nuclei. Its chief
consequence from our point of view is the
restitution of the characteristic chromosome
number.
GAMETE (ga-meet): a marrying cell; the ovum of
the female and the spermatozoon of the mttle.
In the union of the gametes the new individual
and the new generation have their beginnings.
FERTILISATION:
(jeen): the something in the germ cell and
its descendants which is responsible directly
or indirectly for the expression of a given
hereditary character: the unit of organic
inheritance.
GENOTYPE (jeenotype): the actual genetic constitution of an individual: the sum total of the
genes in the hereditary constitution. Also a
group of individuals genetically identical.
GENE
(het-er-o-zy-gus): carrying two
different alleles of a given gene, one on each
chromosome of a pair.
HETEROZYGOUS
, II
8
HOMOLOGOUS (ho-mol-o-gus): homologous chrom?somes are partners, the two members of a
pan.
HOMOZYGOUS (ho-mo-zy-gus): carrying the same
allele of a given gene on both members of a
pair of homologous chromosomes.
HYBRID: the offspring of two dissimilar parents.
LINKAGE: the tendency on the part of certain
hereditary characters to remain together generation after generation; due to the fact that
their genes are resident on one and the same
chromosome.
Locus (lo-kus): the pla.ce in a chromosome
occupied by a given gene.
MATURATION: the series of changes, including
one or more cell divisions which the gametes
undergo before they are ready for fertilisation.
MENDEL'S LAWS: the statements that the factors
of heredity segregate one from the other, and
that the factors belonging to different pairs are
independently di.stributed to the gametes.
MONOHYBRID: the offspring of two individuals
which differ one from the other in respect of
the members of one pair of allelomorphic
characters.
MUTATION: the inception of a heritable variation;
that alteration in the constitution of a gene
which gives rise to a new allele.
GLOSSARY
119
failure of daughter chromosomes to separate in cell division, both halves
going to the same daughter cell. Also failure
of the maternal and paternal chromosomes of a
pair to separate during the maturation of the
gamete so that both chromosomes go to the
same cell.
NON-DISJUNCTION:
(nu-kli-us): that portion of the cell in
which the chromosomes are contained.
NUCLEUS
OVUM:
the gamete elaborated by the female.
(fe-no-type): the sum total of the
hereditary characters apparent in an individual.
Also a group of individuals all of which look
alike.
PHAENOTYPE
(pro-to-plazm): the living stuff of
which animals and plants are constituted.
PROTOPLASM
not being apparent except when
present in the duplex state; incapable of being
expressed when the gene for the allelomorphic
character is also present.
RECESSIVE:
those chromosomes in respect
of which male and female differ; the male has
two X -chromosomes, the female one X and
a Y.
SEX CHROMOSOMES:
association of a hereditary character
with sex due to the fact that its gene is resident
in the X-chromosome.
SEX LINKAGE:
(sper-ma-to-zo'on): the gamete
elaborated by the male.
SPERMATOZOON
120
'THE GENE'I'ICS OF 'THE BUDGERIGA R
an individual or a generation
produced by parents differing one from the
other in respect of the members of three
different allelomorphic pairs of hereditary
characters.
TRIHYBRID:
(zi-gote): the fertilised egg; the individual
having its origin in this.
ZYGOTE
BIBLIOGRAPHY.
Armour, M. D. S. In-breeding tn Bztdgerzgars for Type
and Quality. London: "Cage Bird Fancy."
Price 1/2.
Duncker, H. Book of Budgerigar Matings. London:
The Budgerigar Society. Price 3/2.
Rogers, Cyril. Budgerigars and how to breed cznnamonwings. London: "Cage Bird Fancy." Pnce 1/6.
Steiner, H. Vererbttngsstudien am Wellensittich M elops~tttcus undulatus (Shaw).
Zurich: Archiv der
Julius Klaus Stiftung fur Vererbungsforschung,
Sozialanthropologie und Rassenhygiene. 1932,
VII 1/2. Price 16/-.
Watmough, W. The Cult of the Budgerigar. London:
"Cage Birds." Price 6/-.
Weston, Denys.
The Budgerigar zn Captivity.
London: "Cage Birds." Price 2/-.
INDEX.
Albino, 82.
Allelomorph, 11, 39.
Autosomal colour mosaics, 74.
Blue, 19.
Buttercup, 82.
Cell division, 8, 65, 69.
Cinnamon, 55.
Chromosomes of Budgerigar, 9.
Chromosome elimination, '69.
Criss-cross inheritance, 59.
Crossing-over, 53, 94.
Crossing-over value, 54.
Dark, 14, 43.
Dark greens, Types I and II, 44.
Fallow, 82.
Fertilisation, 9, 65.
Green, 14.
Greywing, 38.
Greywing yellow compound, 40.
Halfsider, 64.
Heterozygote, tests for, 24.
In-breeding, 61, 83, 101.
Independent assortment, 30, 37, 95.
Line breedmg, 101.
Linkage, 42, 92.
Linkage groups, 62.
Lutino, 82.
Matings:Cobalt X cobalt, 43, 44.
Cinnamon X non-cinnamon, 58.
IZI
I:U
INDEX-continued.
Dark green X cobalt, 48.
Dark green, 17,43.
light green, 15.
olive, 19.
Dark yellow X Dark yellow, 43.
LIght green X light green, 19.
light yellow, 25.
mauve, 46.
skyblue, 20.
Olive X light green, 49.
light yellow, 48.
olive, 19.
sky blue , 45.
White X blue, 34.
light green, 34.
yellow, 34.
Yellow X blue, 26.
light green, 25.
Yellow olive X light green, 49.
Mechanism of heredity, 8.
Modifying genes, 14, 83.
Mutation, 2, 10, 38, 61, 72.
Autosomal dominant, 61, 90.
recessive, 61, 90.
Sex-linked dominant, 61, 92.
receSSIve, 59, 89, 91.
Non-disjunction, 72, 76.
Out-breeding, 102, 107.
Pied, 81.
Polyspermy, 68.
Selection, 4, 82.
Sex chromosomes, 12, 55.
linkage, 58.
reversal, 75.
White, 26.
Yellow, 25.

the genetics of the budgerigar

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