Mendel`s Law of Heredity

Transcription

Mendel's Law of Heredity
Author(s): W. E. Castle
Source: Science, New Series, Vol. 18, No. 456 (Sep. 25, 1903), pp. 396-406
Published by: American Association for the Advancement of Science
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396
SCIENGZD.
the giving up of the idea of relying upon
private effort.
That we lose most where the State does
least is known to Mr. Chamberlain,for in
his speeeh, to whiGhI have referred,on the
University of Birmingham,he said: "As
the importaneeof tsheaim we are pursuing
betomesmore and moreimpressedupon the
z>ninds
of the people, we may find that we
sha]l be more generously treated by the
State."
IJaterstill, on the oteasion of a visit to
University College Sehool, Mr. Chamberlain spoke as follows:
"When we are spending,as we are, many
mi]lions I think it is 13,000,0001. a year
on primary edutation, it tertainly see-lasas
i-f we might add a little more, even a few
tens of thousands,to what we give to University and setondary edutation" ( Ttrres,
November6>1902).
To eompeteon equal grounds with other
nations we must have more universities.
But this is not all we want a far better
endowmbntof all the existing ones, not forgetting better opportunitiesfor researehon
the part of both professors and students.
Another erying need is that of more pro:Xessors
and better pay. Another is the reduetion of fees; they should be redueed to
the level in those eountrieswhith are eompeting with us, to say, one-fifth of their
present rates, so as to enablemore students
in the setondary and teehnieal sehools to
Gompletetheir edueation.
In all these ways, faeilities would be afforded for providing the highest instruetion to a mueh greater numberof students.
At present there are almost as many professors and tnstructors in t-heuniversities
and eolleges of the United States as there
are day students in the universitiesand eollegeswofthe United Kingdom.
Men of seienee, our leaders of industry,
and the ehiefs of our politieal parties all
agree that our present want of higher edu-
LN S.
VOL.
XVIII. No. 4*56.
eation in other words, properly equipped
universities-is heavily handiGappingus in
the present raGefor tommereialsuprematy,
beeause it provides a relatively inferior
brain-powerwhiGhis leading to a relatively
redueed national intome.
The faets show that in this eountry we
ean not depend upon private effort to put
matters right. How about loGaleffort?
Anyonewho studies the statisties of modern muniGipalitieswill see that it is impossible for them to raise rates for the
building and upkeep of universities.
The buildings of the most modern university in Germany have Gost a million.
DIfor
upkeep the yearly sums found, ehiefly
by the State, for German universities of
different grades, taking the intomes of
seven out of the twenty-two universities
as examples,are:
1st Class............
2nd Class...........
,
130,000
Berlin
f Bonn
)< Gottingen
3rd Class............
f Konigsbelg
4th Class...........
f
i
Strassburg
Heidelberg
Marburg
A
J
t
J
t
J
56,000
48,000
37,000
Thus if Ijeeds, whieh is to have a university, is eontene with the 4th elass German standard, a rate must be levied of 7d.
in the pound for yearly expenses,independent of all buildings. But the faets are that
our towns are already at the breaking
strain. During the last £ty years, in spite
of enormous inereases in rateable values,
the rates have gone up from about 2s. to
about 7s. in the pound for real local purposes. But no university ean he a merely
loeal institution.
NORMAN
I2OCKYER.
( To be concl?bded.)
MlSNDlSL'S LAW 0F HEREDITY.t-
WHATwill doubtless rank as one of the
great diseoveries in biology, and in the
*tThis paper was originally published in part in
tile Proceedings of the Anterican Academy of Arts
aznd Sciencesa Vol. 38, No. 18, pp. 535-548, January, 1903.
25, 1903.]
SEPTEMBER
SGIENCE.
study of heredity perhaps the greatest,was
made by GregorMendel,an Austrian monk,
in the garden of his cloister, some forty
years ago. The diseovery was announeed
in the proeeedings of a fairly well-known
seientifie soeiety, but seems to have attraeted little attention and to have been
soon forgotten. The Darwinian theory
then oeeupied the eenter of the seientifie
stage and Mendel's brilliant diseovery was
all but unnotieed for a third of a century. Meanwhilethe diseussionarousedby
Weismann's-germ-plasmtheory, in particular the idea'of the non-inheritaneeof aequired eharaeters, had- put the; seientifie
public into a more receptive frame of
mind. Mendel's law was rediscoveredindependently by three different botanists
engaged in the study of plant-hybrids-de
Vries, C'orrens and Tschermak-in the
yeSar1900. It- remained, however, for a
zoologist,Bateson, txvoyears later, to point
out the full importanceand the wide applicability of the law. Since then the Mendelian diseoverieshave attracted the'attentionzof biologists generally. Accordinglya
brief statement of'their underlying principles maaynot be without interest to others
also.' 7
'
1.
T}ie
Law
of
Domina?ce.
When
. .
.
..
mating oceurs between two animals or
tplantsdiffering in some character,the off'springfrequently all exhibit the character
of one parent only, in which case that
character is said to be 'dominant.'
Thus,
when white mice are crossed--w-ith gray
mice, all the offspring are gray, that color
characterbeing dominant. The character
which is not seen 'in- the immediate ofEspring is called;recesstre,' for though unseen it is still present in the young, as we
shall see. White, in the instance given, is
the recessive tharacter. The principle of
heredityjust stated may be called the law
-ofdomtnance. The first instance of it dis:coxrered
hy Mendel related to the Gotyle..
.
.
.
.
397
d.on-color.
in peas obtainedby crossing different garden varieties. Yellow color of
eotyledons was found to be dominant over
green; likewise, round smoothform of seed
was found to be dominant over angular
wrinkled form; and violet eolor of blossoms, over white eolor. Other illustrations
might be mentioned both among animals
and amongplants, but these will suffice.
2. Peculiar Z[ybqxdFornts.-The law of
dominangeeis not of universal applieability; Mendel does not so deelare, though
some of his erities have thus interpreted
him. In many eases the eross-br.edoSspring possess a eharacter intermediate
betweenthose of the parents. This Mendel
found to be true when varieties of peas
differing in height were crossed.
Ag.ainJthe cross-bredsmay possess what
appearsto be aqttntensficatton of the character of one parent, as when in crossing
dwarf with tall peas the hybrid plant is
taller than etther nparent,or as when,. in
cressing a brown-seeded.with a whiteseeded variety of bean, the ofEspringbear
beans.of a darker brown than those of the
brown-seededparent.
. ...
..
Thirdly, the cross-bredmay have a characte.rentirely different from that of either
p.arent. Thus a cross between spotted,
black-and-white mice, and albino -miee,
prod.u.e.es
commonl.ymice entirely.gray in
color, like the house-mouse. ..Again in
crossing beans, a var.ietyhaving yellowishb.romrnseeds crossed with a white-seeded
variety yields sometimes black-mottled
seed,. a char.acter possessed by neither
parent.
These three conditions may be grouped
together by saying-the hybrid often possesses a characterof tts own, instead of the
pure tharacter of one par.ent,as is true in
cases of complete dominance. The hybrid
ch.aractermay approximate that of one
parent or the other, or it may be different
from both.- There is no way of predieting
398
[N. S.
SCIENCE.
vOL.
XVIII. No. 456.
what the hybrid cha-racterin a-gi-ven-cross are..possible,sinee each parent will £urnis.h
will be. It can be determinedonly by e'x- AsSa.ndB.s in equal numbers. The possible
are AA, AB and.BB. Theperiment,but it is. always the same £or the co.mbinat.ions
'same cross, providedthe parents are pure. first.sort will.'consist of pure As and will
'Often the hybrid fornl resembles aNsup- breed true to that character ever aft.er-S;
posed ancestral condition, in which case ;ward, unless crossed with individuals
it is eommonlyd'esignateda reversion. -I1- having a d:ifferent-charaeter...Similarly,
lustrationsare the gray hybrid mice,which- the. third-sort will consist of pure Bs. and;
are indistinguishablein appearane-et
from will breed.true to that character.; But the
the'house-mouse,and slate-coloredpigeons second sort, AB, will consist of hybrid inresulting from crossing'white- w-ith buff dividuals, like those of 'which the first hyplxgeons.
,r r
brid generation was excIusively compo'sed.
3. Pnty of the Germ-cells.-The great- I.f, as supposed, germ-cells,'A and B, are
discovery of Mendel is this: The hybrtd7 produced in equal numbersby hybrids of
whatever its own character,produces rtpe both seses, and unite at randomin fertiligerm-cellswhich bear only the pure char- zation, combinations AA, AB and B13
' acter of orte parent or the other. Thus, should) occur "in the frequencies, 1: 2:1.
when one parent has the characteraA,an'd Florin un.ionsbetween.t.wosets-of gametes,
the other the character.B, the'jhybridwill each A+B, there is one chance each:for
have the characterAB, or in cases of simple the combinations AA and BB, but two
'dominance,A(B')* or B(A). But-what- chances for the combinationAB. .
ever the character"ofthe hybrid may be,
If the three forms AA (.or;simply A),
its germ-cells, when- mature, will bear AB and'B are all different in appearance,
either the characterA or the characterB, i.t will be a very simple matter.in an exbut not both,;and As and Bs will be'pro- periment to count those of each class and
dueed in eqxal nurnbers. This perfectlfir determinewhether they occur in the theosimple principle is known as the law of *reticaIproportions,1: 2: 1. One such case
'segregatton,',or the law of the 'purity of has been observedby Bateson ( :02, p. 183)
the germ-cells.' It bids £air to prove as among Chinese primroses (Pqxmxlastnenfundamental to a right understanding of sts). An unfixabl.ehybrid variety known
the facts of heredity as is tlie law of defi- as ' giant lavender,' bearing flowers of a
nite proportions in chemistry. F'rom it lavender color, was produced by crossin¢
'ollow many important consequences.
TABLE I.
- A first consequenceof the law of purity
of the germ-eells is polymorphismof the
Characters,
,",^,
>
AB.
second and later hybrid generations. The
Magenta
LavWhite.
vlantsbearingFlowersin Color Red.
ender.
individuals of the first'hybrid'generation
are all of one type, provided the parent 1901, Lot 1..............
14
27
1901, Lot 2..............
9
9
0
individuals were pure. Each has a char- 1902,
Lot 1..............
12
23
acter resulting from the combination of 1902, Lot 2 .... .. .. ......
11
14
26
an A with a B, let us say AB. [In cases
45
Totals .................
54
96
22
Per cent. of whole.....
29
49+
of clominanceit would moreproperlybe expressed by A (B) or B (A)-.] But in the
next generationthree sorts of combinations a. m.agenta red with a white flowering
variety tinge.dfaintly with pink.. By seed
4t The parenthesis is used to indicate a recesthe hybrid constantly . produces plants
sive character not visible in the individual.
*
.
.
.
.
.
.
.
.
.
B.
A.
19
S
11
Y
G
L
1G
1Y
G 1G
Y
x'
... GY G
lW1G ..
GW
SCIENCE.
SEPTEMBER 25, 1903.]
399
preceding generation. These are precisely
the theoretical proportions, A 4- 2 A(B)
+ B.
In the case of mice, it has been shown
independently by Cuenot ( :02) and by
the writer's pupil, Mr. G. M. Allen, that
the second hybrid generation, obtained by
crossing gray with white mice, consists of
gray mice and white miee approsimately
in the ratio 3: 1. ( See Table III. ) The
white are pure recessives, producing orlly
TABLE II.
white oftspring,when bred tnter se. What
HEREDITYOF COTYIEDON
COLOR
AMONGC:ROSS-BREDportion of the gravs are pure dominants
PEAS.
\
has llot yet been determinedwith precision,
but we may confidentlyexpect that it will
Parents
OfSpring.
cro9sed
Gen. I.
Gen. II.
-Gen. 1II.
Gen.IV.
prove to be not far from 1 in 3.
bearing magenta red and white flowers
respectively as well as other plants bearing lavender flowers. The numerical proportions observedin two successiveseasons
are shown in Table I. The observednumbers, it will be seen, are quite close to the
theoretical 1: 2: 1.
In cases wherein the hybrid is indistinguishablefrom one of the parent for-ms,
i. e., in cases of complete dominance of
TABLE III.
F
Y(G)
{2Y(G){
342Y(G)
HEREDITYOF (:eAT-COI.OR
AMONGCROSSBREDMICE
OnTAINFDBY MATINGWHITE MICE ( W )
WITH GRAY MICE ( G ) .
l
one parental character, only two categories of of}'spring will be recognizable
and these will be numericallyas 3: 1. But
further breeding will allow the separation
of the larger group into two subordinate
classes-first individuals bearing only the
domirlant character; secondly, hybrids;
that iS7 into groups A and A(B), which
will be rlumericallyas 1: 2.
Observed results are in this case also
very close to theory. Mendel, by crossing yellow with green peas, obtained, as
we have seen, only yellow (hybrid) seed.
Plants raised from this seed bore in the
same pods both yel]ow seed bnd green seed
in the ratio 3: 1. (See Table II.) Under
self-fertilization, the green seed produced
in later generations green seed only. It
bore only the recessive character. Of the
yellow seeds, one in three produced on]y
yellow oWspring,. e., contained only the
clominantcharacter; but two out of three
proved to be hybrid, producing both green
and yellow seed, as did the hybrids of the
Parents
Crossed.
l
Offspring.
Gen. II.
Gen. I.
3{
¢en. III.
1W
3 { 2G(W)
GJ
A further test of the correctness of
gendel's hypothesis of the purity of the
germ-cellsand of their productionin equal
numbers, is afforded by back-crossingof
a hybrid with one of the parental forms.
For example, take a case of simple dominance7 as of cotyledon-color in peas or
coat-color in mice. We have here characters D (dominant) and R (recessive).
The first generation hybrids will all be
D (R) . Any orle of them back-erossed
with the recessiveparent will producefifty
per eent. of pure recessive offspring and
fift-yper cent. of hybrids.
D+ R
For the hybrid produces germ-cells
The recessive parent producesgermR+ R
cells .........................
2D(R) + 2R
The possible combina,tionsa,re....
400
SCIENCE.
[N. S. VOL.XVIII. NO.456.
This ease has been testedffor peas and Goatpresents a eurious rough appearance,
for miee and found to be substantially as for the reason that the hair stands out
stiffly from the body in a number of 'eowstated.
V9'ehave thus far eonsideredonly eases lieks' or rosettes. In erossesthe Abyssinian
of eross-breedingbetween parents differ- or rough eoat regularly dominates over
ing in a single eharaeter. We have seen the norrnal or smooth eoat. Now let us
that in sueh eases, no new forms exeept eonsider what happens when a eross is
the unstable hybrid form, are produeed. made involving both these pairs of Men:13utwhen the parent forms erossed differ delian eharaeters, albinism vs. pigmented
in two or more eharaetersnthere will be eoat, and smooth vs. rough eoat. If a
produeed in the seeond and later hybrid white Abyssinian is bred to a pigmented
generations individuals possessing new smooth guinea-pig, the young are without
of the eharaeters found in exeeption pigtnented and rough, these
counbtqtattons
all posstble cornbtna- being the domtnant members of thevtwo
indeed,
the parents;
ttons of those eharaeters will be formed, pairs of eharaeters. But the young of
and in the proportions demanded by eourse eontain in a latent condition the
ehanee. Thus when parents are erossed two reeessive eharaeters, white eoat and
whieh differ in two respeets, A and B, smooth eoat, whieh faet may be indicated
let us designate the dominant phase o:f by designating them as already suggested,
these eharaeters by A, B, the reeessive AB(ab) [A3 a referring to the rough or
phase by a, b. The immediate oftfspring
smooth eharaeter of the eoat and B, b to
resulting from the eross will all be alike,
its eolor].
AB(ab),4; but the seeond and later generaThese primary hybrids, if bred tnter SG,
tions of hybrids will eontairlthe stable, .
of four different sorts,
e., pure elasses, AB, Ab, aB, ab, in addi- will produee young
rough pigmented, routgh white,
tion to other (unstable or still hybrid) viz.,
pigmented and smooth white. A
smooth
forms, namely, A13(ab), AB(b), A(a)B,
number of the animals of eaeh
A(a)b and aB(b). In every sixteen eertain
breed true, . e., will produeeonly
will
sort
seeond-generatio:noffspring there will be,
sort when mated to animals like
own
their
on the average, one representing eaeh of
Theoreticallythere should be
themselves.
the stable eombinations. Two of the stable
pure individual of eaeh of the four
one
eombinations wi]:l be identieal with the
sorts in a total of sixteen young. The
parent forms,.the other two will be new.
pure individuals answerto the elasses
The remaining twelve individuals will be four
Ab, aB, ab already rnentioned.
AB,
hybrid in one or both eharaeters.
>)esidesthese pure individuals,there
But,
An illustration may help to make this
in three of the four elasses
oeeur
will
ease elear. Among domestieated guineaindividuals, whieh will
hybrid
or
tmpxre
pigs, as among miee and rabbits, albinism
their young the domiof
some
to
transmit
is recessivewbthrespeet to pigmentedeoat.
whieh they
eharaeters
or
eharaeter
nant
Further, there oceur amongguinea-pigsinto others of their
but
possess,
themselves
dividuals known as 'Abyssinians,' whose
young the eorresponding reeessiareehar* This is Mendel's use of lower-case letters to aeter or charaeters. Only the class of
I have
designate recessive characters, with +^7hich
smooth white animals ( of which there
combinedthe use of a parenthesis when a character by nature recessive is not visible in the indi- should be one in sixteen young) eontains
none but. pure individuals, for they bear
vidual.
SEPTEMBER 25, 1903.
]
SCIENCE.
the two reeessive eharaeters (ab), and so
eoneealno hidden reeessives. Thewr
may at
onee be set aside as pure. But in the other
three elasses nothing but aetual breeding
tests will serve to show whieh individuals
>arepure and whieh impure or hybrid. To
eaeh pure inclividualpossessing one dominant and one reeessive eharaeter there
will be two others, exactly like tt tn appearance, but hybrid in one pair of eharaeters. This statement applies to the two
elasses, rough-white and smooth-pigmented, in whieh the impure individuals
would be designated A(a)b and aB(b)
respeetively. Sueh impure animals bred
tnter se would produee, in the ease of
rough-whiteparents, both rough-whiteand
smooth-whiteoflSspring,and in the ease of
smooth-pigmentedparents, both smoothpigmented and smooth-whiteoffspring.
In the elass of rough-pigmentedseeondgeneration oSspring, whieh eombines the
two dominant eharaeters,there will be to
eaeh pure individual ( AB ) eight whieh
are impure in one or both charaeters. Two
of the eight will be hybrid in one charaeter
only, as in the rough vs. smooth eharaeter they form the elass A(a)B; two
other individuals will be hybrid in the
other eharaeter? albino vs. pigmented,
Ieormingthe elaXssAl3(b); while the remaining four will be hybrid in bot7?eharaeters, exaetly like. the entire first generation of oifspring, AB (ab) .
The task of the praetieal breeder who
seeks to 'establish' or 'fix' a new variety,
produeedbyeross-breeding,ina ease involving two variable eharaeters,is simply the
isolation and propagation of that one in
eaeh sixteen of the seeond-generationoffspring whieh will be pure as regards the
desired eombination of eharaeters. Mendel's diseovery by putting the breeder in
possessionof this information enables him
to attaek his problem systematieally, with
401
eonfideneein the outeome,whereasthitherto
his worlLd,
important'and faseinating as it
is, has eonsisted largely of groping for a
treasure in the dark.'
The greater the number of steparately
variable eharaetesrsinvolved in a eross, the
greaterwill be the nulnberof new eolnbinations obtainable; the greater, too, will be
the numberof individuals whieh it will be
neeessary to raise in order to seeure all
the possible eombinations;and the greater,
again, will be the diffieultyof isolating the
pure, t. e., stable forms from sueh as are
similar to them in appearanee but still
hybrid in one or more eharaeters. Mendel
has generalized these-statements substantially ' as follows: In -eases of eomplete
dominanee,when the numberof differenees
between the parents is n, the number of
different elasses into whieh the seeond generation of offspring fall will be 3nSof
whith 2n will be pure (stable); the remainder will be hybrid, though indistinguishable from pure individuals. The
smallest number of individuals whieh in
the seeond hybrid generationwill allow of
one pure tndividual to eaeh visibly different elass will be 49. (See Table IV.)
.
TABLE IV.
a
Z
5 ;l 5
,
F |
E
<
n
Sn
3n
4n
1
2
3
2
4
8
3
9
27
4
16
- 64
4
5
6
16
32
64
81
243
729
256
1024
4096
°
)
Tested byMendel
Wforpeasand found
J
correct.
Calculated.
The law of Mendelreduees to an exaet
seienee the art of breedingin the ease most
earefully studied by him, that of entire
402
seIsNas.
It gives to the breeder a new
dominance.
of ' purity. ' No animal or
aonception
deis 'pure' simply because it is
plant
posancestors
of
from a long line
scended
of characa
sessingdesired combination
if it
but ay animal or plant is pure
ters;
even
gametes of only one sort,
produces
its-grandparentsmay among themth'ough
have possessed oppostte characters.
selares
existence of purity ean be established
The
breeding
ithcertainty only by suitable
M
receswith
tests(especially by crossing
for
but it may be safely assumed
sives),
paanimal or plant descended from
any
had
which were like each other and
rents
shown'by breedingtests to be pure.
been
1Wendel.
Special Cases nder the Law of
cases
special
-Stremains to speak of some
apparently
the law of Mendel,which
under
of the
areexceptions to one or another
already stated, and which probprinciples
conditions
ablyresult from exceptiona]
sp'ecial
These
imperfectly.
linownto us
through
caseshave come to light in part
that
through
part
in
Mendel'sown ^ork,
ofothers.
It oeeasionally
1. lMosaic InhUerwtance.
bring towhieh
erosses
happensthat in
reeommonly
gethera pair of eharaeters
two
the
reeessive,
and
latedas dominant
in
eharaeters appear in the offspring
animals
piebald
in
as
patchesside by side,
The
andparti-eoloredflowers and fruits.
plaee
gives
normaldominance apparently
in such eases to a balaneed relationship
What
betweenthe alternative charaeters.
is
relationships
eonditionsgive rise to sueh
seeured
onee
are
unknown,but when they
stability,
they often prove to possess great
example,
for
This,
breedingtrue tnter se.
usually
whieh
is the ease in spotted-miee,
offspotted
of
produee a large majority
eharof
relationship
balanced
spring CL'he
is transaeters possessed by the parents
not as
are,
whieh
mitted to the germ-eells,
S. VOL.XVIII.
[N.
NO. 456.
D or R,
inordinaryhybrid individuals
the
to
DR. This has been shown be
but
myand
in spotted miee by Mr. Allen
ease
in a paper published elsewhere.
self,
and Allen, : 03. )
(Castle
ease,
2.Stble XybridForms.-This is a
whieh
somerespeets similar to the last,
in
It
familiar to Atendel ( :70) himself.
was
that
happens, as we have seen,
sometimes
hybrid has a form of its own diSerent
the
eases
that of either parent. To sueh
from
not
does
law of dominanee evidently
the
hyHteraciuen
apply. In a few eases
(Wiehura)
(Mendel), Salix hybrids
brids
form
--ithas been found that the hybrid
not break up in the seeond generation
does
grandandproduee individuals like the
hybrid
own
its
to
true
but breeds
parents,
on
only
explained
eharaeter.This ean be
germthe
Either
oneof two assumptions.
the balcellsbear the two eharaeters in
of
those
aneedrelationship, AB? as do
whieh
spottedmiee, or, of the two gametes
bears
nnitein fertilization, one invariably
eharaeter
the
other
the eharaeter A, the
former
B. Of the two explanations, the
probable.
more
seemsat present mueh the
phe3. Co?pledChclracters.-Thisis the in
eharacters
of
nomenonof eorrelation
that, in
heredity. It is sometimesfound
not be
ean
eross-breedina,two eharaeters
the
separated. NVhen one is inherited,
crossin
other is inherited also. Thus,
Jamesing diflSerentsorts of Datura (the
purple
town weed) it has been found that blue
with
eolor of stem invariably goes
stems are
green
whereas
eolor of flowers,
flowers.
eonstantly assoeiated with white other
most
Again in lrliee, rabbits and
eyes eommammals,white hair and pink
not be
monly oeeur together and may
howseparated in heredity. Very rarely, perever, as I have observed,an otherslTise
eyes i
feetly mrhiteguinea-pig has dark
guinea-pi
further the ordinary albino
SEPTEMBER - 25,
1903.]
403
SCIBXCE.
-
\
-
with pink eyes has usual]y smutty (brown- -havebeen shown,-by breeding tests, to be
pigmented) ears, nose and feet; and a race hybrids, sinee on crossing with white mice
of miee with pink eyes, though partially they produce white miee, black mice, and,
pigmented eoat, has formed the basis of in one or two eases, gray mice also. Aesome reeent importantsexperimentsin he- eordingly blaek mice elearly belong with
sredityeonduetedby Darbishire (: 02,: 03) grays in the category of dominantindividat Oxford, England. The$e exeeptional uals [D or D (R)], but they-have visibly
conditions probably represent stable coup- only the black constituent of the gray eoat,
lings of a part only of th@dominantschar- the remaining eonstituent, a rufous tint,
acter (pigmented eoat) with the reeessive having been separated from the blaek in
charaeter (white coat), and are similar in eonsequeneeof eross-breeding. There is
kind to the DR eharaeterof spotted mice. reason to believe that the rufous constiFurther, eoup]ing may oeeur between a tuent may become reeessive, . e., latent,
in the
number of charaeters greater than two, either in the blaek individuals or
seen
is
It
both.
in
or
whites,
reverted
so that they form, to al] intents and purthe
and
black
the
both
from
separated
poses, in heredity, one indissoluble comehocolate-brown
the
in
pound eharacter. Thus, Gorrens (:00 ) white eharacters,
individuals obtained
observedthat in erossesbetweentwo speeies and reddish-yellow
in cross-breeding.
of stocks (lliathtola ineana DC. and M.
A faneier of rabbits tells me that there
glabra DC.) the seeond generationhybrids
oeeurs a similar disintegrationof the eomshowedreversionto one or the other of the
coat-eolor of the ' Belgian hare,'
parental forms in ata three of t}e princtpal posite
animal is crossedwith ordinary
that
when
dtgereqtttatcharacters studied, viz., hairy
the result being the producrabbits,
white
or glabrous stems, violet or yellow-white
yellow and mottled indi.black,
of
tion
flowers, and blue or yellow seed. A blue
to ordinary grayaddition
in
viduals,
seed always produced a hoary plant bearing violet flowers; a yellows seed always browns.
The various distinct colors or color
produced a glabrous plant bearing yellow
patches of the guinea-pig have doubtless
or white f owers.
originated in a similar way by resolution
is
This
4. Disintegrattonof Characters.
of the composite coat-color of the wild
Not
process.
the converseof the foregoing
Cavta, upon crossing with an albino sport.
be
simple
only may charactersapparently
subject is now undergoing investigaThis
comcouple(l together in heredity to form
tion.
posite 1lnits o£ a higher order, but characCorrens ( :00) mentionsa case in plants,
ters which ordinarily behave as units may
as a result of crossing undergo disintegra- which probably belongs in this same catetion into elementsseparately transmissible. gory. In crossing the blue-flowered(domtncana with the yellowishThus the gray coat-color of the house- inant) 7[Iath>zolcl,
mouse is always transmitted as a domin- white-flowered (recessive) M. glabraf the
ant unit in primary crosses with its white second generation recessives produced in
variety; but in the second cross-bredgen- some cases pure white flowers, in others
eration a certain number of black mice ap- yellow flowers. In this case the recessive
pear, some or all of which are probably aharacter, rather than the dominant, unhybrids. For similar black mice obtained derwent disintegration.
5. Departures from t7se Theorettcal
by crossing black-white with white mice
40
SCIENCE.
[N- S.
VOL.
XVIII. No. 456.
It is possible, however, that there are
Rattos of Dornants to Recessq>ves.Considerable departures are to be expected cases in which one of a pair of characters
when the number of oflfspringtaken into is sometimes doninant, sometimes reeestonsiderationis small, but with increasein sive. Tschermak ( :01) believes that he
the number of offspring examined,the de- has found a few such cases among crosspartures should grow less. This is usually bred beans. Sex and certain other difound to be true. Mendel's numbers are morphic conditions found in the higher
shown by Weldon ( :02) to be well within animals and plants may prove to be cases
the limits of probable error. But certain of this sort.
Acceptance of Mendel's prineiples of
cases have been observedin which departheredity
as correctmust lead one to regard
ures of a particular sort persist even with
discontinuous
( or sport) variation as of
considerablenumbers of offspring. Thus
Allen and I have found the recessive char- the highest importanee in bringing about
acter,white, in mice to be inherited'inabout polymorphismof species and ultiluately of
\
three per cent. more than the caltulated theaformationof new species.
A sport having once arisen affecting
number of cases, while the equally recessive dancing characteris inherited'in about some one character of a species, may by
thirty-three per 'cent. less than t-he caleu- crossing with the parent form be the cause
lated number of cases. These fairly uni- of no end of disintegrationon the part of
form departures indica;te,to my,mind, a any or all of the charactersof the species,
vitality, on the part of the recessiare and the disintegrated characters may, ingamete, in one case somewhatsuperior, in deed ntest, form a great variety of nemr
the other much inferior, to that of the Gombinationsof characters,some of which
dominant gamete. Inferior vitality of will prove stable and self-perpetuating.
if a particular combinationof ehargametes of either -sort xvould result in DE+ren
greater rLlortalityand so in a diminished acters is uniformly eliminated by natural
number of individuals derived from such selection under one set of conditions, it
ulay reappear agairl and again, and finally
gametes.
^
Of courseother explanationsare possible, meet with conditions which insure lts
as, that the two sorts of gametes are not sueeess.
We now have an explanationof the lonproduced in equal numbers. More extended investigations of such cases can reeognizedprinciple that new types of organisms are extremely varia-ble,whereas
alone make their meaning elear.
6. Reversal ot7Dowtnarbce. Exeeptional old types vary little. A new type which
cases are on record in which trossing of a has arisen as a sport will cross with the
dominant with a reeessive has resulted in parent form. The offspring will then inthe production of pure dominants, or re- herit some characters dominant, others
cessives, instead of hybrids. Sueh cases latent, and polymorphismof the race reare, I believe, eorrectly referred by Bate- sults. Only selection continued through
son to the category of ' false hybridiza- long periods of time will serve to eliminate
tion' as deseribed by Millardet, a phe- conzpletelythe latent recessives,and so to
nomenonakin to parthenogenesis,in which eause the disappearance of certain abersesual union has served rnerely to sttrnts- rant variations.
Bateson makes the pregnant suggestion
late one gaqzete to developm,entwithout
that
even eases of coritinuous variation
bringing about its union with the other
may
possibly
prove eonformablewith AIengamete.
SEPTEX1BER25, 1903.]
SCIEEGE.
delian prineiples. Take, for example, the
height of peas. It. has been found in certain crosses of a tall with a dwarf variety
of pea, that the hybrid has an intermediate height. Now, if the hybrid produe.espure germ-eells, dwarf and tall respeetively, in equal numbers,the next generation will consist of three elasses of
individuals, dwarf, intermediate and tall,
in the proportions1: 2: 1. But if each of
the original charactersshould undergo disintegration, we might get a dozen elasses,
instead of three, resulting in a practically
continuous frequeney-of-erroreurve.
405
4. There htaarefbeen
observedthetfollowing exceptions to the principle of dominance, or to the principle of purity of the
germ-eells,or to both;
(a) Mosaic inheritance,in which a pair
of ellaractersordinarily related a.s dominant and recessive occur in a balanced relationship, side by side in the hybrid individual and frequently in its germ-eells
also. This balanced condition, once obtained, is usually stable under elose breeding, but is readily disturbedby cross-breeding, giving place then to the normal
dominance.
( b) Stable (self-perpetuating) hybrid
SUMMARY.
forms result from certain crosses. These
1. The basie prineiple in Mendel's dis- teonstitutean exeeption to both the law of
eoveries is that of the purity of the germ- dominan,eeand to that of purity of the
eells, in aeeordaneewith this a eross-bred germ-eells. CFlor
the hybrid is like neither
animal or plant produees germ-eellsbear- parent, but the eharaetersof both parents
ing only one of eaeh pair of eharactersin exist in a stable union in the mature germwhieh its parents differ. :lfromit follows eells produeedby the hybrid.
the oeeurrencein the seeond and later hy(c) Coupling, t. e., eomplete ,eorrelabrid generations of a definite number of tion may exist between two or more eharforms in definite numerieal proportions.
aeters, so that they form a eompoundunit
2. Mendel's prineiple of dominanee is not separablein heredity, at least in eertain
realized in the heredity of a eonsiderable erosses.
:numberof eharaetersamong both animals
(d) Disintegration of eharaeters apand plants. In aeeordaneewith this prin- parently simple may take plaee in eonseeiple, hybrid ofEspringhave visibly the quenee of eross-breedin.
eharaeterof only one parent or the other
(e) Departuresfrom the expeeted ratios
though they transmitthose of both parents. of dominants to reeessives may be ex3. In other eases the hybrid has a dis- plained in some eases as due to inferior
tinetive eharaeterof its own. This may ap- vigor, and so greater mortality,on the part
proximate more or less elosely the eharof dominants or recessives respeetively.
aeter of one parent or the other or it may
(/) Cases of apparent reversal of dombe entirely different from both. CFlre- inanee nzayarise from 'false hybridization'
quently the distinetive hybrid charaeter (indueed parth>3nogenesis). Possibly in
resembles a lost aneestral eharaeter. In other eases the determination of domsome eases of this sort, as in eoat-eolorof inallee rests with circumstances as yet
mammals, the hybrid eharaeter probably unknown.
results from a reeombinationof the ehar
5. Mendel's prineiples strengthen the
aeters seen in one or both parents, with view that speeies arise by diseontinuous
certain other eharaeters latent (that is variation. They explain why new types
reeessive) in one parent or the other.
are espeeially variable, how one variation
.
406
[N.S. NToL.XVIII.
saIsNcs.
eausesothers?and why tertain variations
areso persistenl;in their occurrence.
BIBI.IOGRAPHY.
\'V.
BATESON,
:02. ' Mendel's Principles of Heredity, a Defence.' MTith a Translation of Mendel's Original Papers on Hybridization.
12mod212 pp. Cambridge. [England.
Contains bibliography and portrait of
Mendel.]
E. R.
W., ANDSAUNDERS,
BATESONT,
:02. 'Experimental Studies in the Physiology
of Heredity.' Reports to the Evolution
Committee of the Royal Society. Report I., 160 pp. London.
M.
STLE,NV.E., ND ALLEN,GLOVER
CA
:03. 'The Heredity of Albinism.' Proc. Am.
Acad . A rts Sc+., Vol. 38, pp. 603-622.
C.
CORR1BATS,
:00. ' G. Mendel'sRegeln iiber das Verhalten del
Nachkommenschaft der Rassenbastarde.' Ber. dexhtsch. bot. Gesellach.,
Jahrg. 18, pp. 158-168.
L.
CUTENOT,
:02. ' La loi de Alendel et l'heredite, de la
pigmentation chez les souris.' Compt.
Rend., Parisd 'Rom.134, pp. 779-781.
A. D.
DARBISHIRE,
:02. 'Notes on the Results of Crossing Japanese Walt7ing Mice with European A1bino Races.' Btornetrzka, Vol. 2, Pt.
1, pp. 101-104, 4 figs.
A. D.
DAR13ISH1}{E,
:03. ' Second Report on the Results of Crossing Japane3e Waltzing Mice with European Albino Races.' Biornetrika,
Vol. 2, Pt. 2, pp. 165-173, 6 figs.
MENDEI.,G#.
:66. 'Versuche iiber Pflanzenhybriden. Verh.
l\Tat?rf.-97ereins in Brxnn, Bd. 4, Abh.,
pp. 3-47. (Translation in Bateson,
:02. )
MENDEI.,G#.
:70. ' Ueber einige aus lviinstlicher Be£ruchtung
Hieraci¢m-Bastard e.'
entnomennen
tn Brxnn, Bd.
Nat1hrf.-Vereins
Verh.
8, Abh., pp. 26-31. (Translation in
Bateson :02. )
E.
51SCHEPUMAK
:00. ' Ueber kiinstliche Kl euzung bei Pis¢ns
satisvxqn. Zeitsch. f. Iandxwirths. Verslxchs1cese^ i¢ Oester., Jahrg. 3, pp.
46t5_<,55
No. 456we
E.
TSCHERMAK,
:01. ' Weiteze Beitrage uber Velschiedenwerthigkeit der Merkmale bei Kreuzung
von Erbsen und Bohnen.' (Vorlaufige
Mittheilung.) Ber. deutsch. bot. Gesellsch., Jahrg. 19; pp. 3551.
E. DE.
VRIES,
:00. ' Sur la loi de disjonction des hybrides.
Comnpt.Bend., Paris, Tom. 130, ppw
845-847.
H. DE.
VRTES.
:00. ' Das Spaltungsgesetz der Bastarden.
Jahrg;
Ber. de¢tsc7l. bc)t. CZesellse1W.,
83-90.
pp.
1S,
M. F- R77EI.I)ON,
:02. ' Mendel's Laws of Altelnative Illhelitance in Peas.' Biomet}ika, Vol. 1
pp. 228 254, pl. 1, 2.
NV.E. CASTLE.
ITY.
29t\DC,-1\E12S
Hh1tX
_
l l l,B UlB CIjIAtTO\- WNIGHT.
THEsubjeet of this sketch was born on
a farm at Rochelle, Illinois, December13s
1858. Early in his boyhood his parents,
Mr. and Mrs. David A. Knight, removed
to a farm at no great distance from Lineoln, Nebraska. Here he grew to young
manhoodgaining the strength of body ancl
mind whith is so often developedin unfettered eountry life. Self-relianee ancl
strength of tharacter came to him in the
struggle that he, in commonwith the other
nzembersof the family, had plat f(rth in
what was then the new west, in order to
wrest from nature the daily bread. Life
in all of its foruls, and the hills an=lr)eks
appealed strongly to him. By the time
that he had secured such education as the
he had also become
eountry school alcEorded
more than ordinarily familiar with the
£auna,the flora and the geological fornlaLionsof his neighborhood.
Being unusually £ond o£ athletie sportst
of fishing and of hu;rlting,he led many a
merry party in these pursuits, frequently
to the complete exhaustion of most of his
fellows. In more recent years, his many
friends who at one time or another shared

Mendel`s Law of Heredity

Transcription

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