2 Mendelismo

2 Mendelismo
mount of pigwings. If the
the wings are
pigment, the
d), geneticists
contain some
h light wings.
The butterflies
f they happen
gs would help
a dark surface
wings would
y lit meadow.
notice a lightA population
find that the
and the light-
F I G U R E 1 . 8 Two dyeing poison frogs (Dendrobates tinctorius)
showing different morphs within a single species.
2
good model genetic organism? Why or
domestication of plants and animals, which began between
approximately 10,000 and 12,000 years ago in the Middle
East. The first domesticated organisms included wheat, peas
lentils, barley, dogs, goats, and sheep (Figure 1.9a). By 4000
years ago, sophisticated genetic techniques were already in
cient peoples practiced genetic techniques in agriculture. (Left) Modern wheat, with larger
e numerous seeds that do not scatter before harvest, was produced by interbreeding at least
ferent wild species. (Right) Assyrian bas-relief sculpture showing artificial pollination of date palms
me of King Assurnasirpalli II, who reigned from 883 to 859 B.C. [Left: Scott Bauer/ARS/USDA. Right:
opolitan Museum of Art, gift of John D. Rockefeller, Jr., 1932. (32.143.3) Photograph © 1996
itan Museum of Art.]
ced genetic techniques in agriculture. (Left) Modern wheat, with larger
t do not scatter before harvest, was produced by interbreeding at least
ight) Assyrian bas-relief sculpture showing artificial pollination of date palms
(a) Pangenesis concept
(b) Germ-plasm theory
1 According to the pangenesis
concept, genetic information
from different parts of the
body…
1 According to the germ-plasm
theory, germ-line tissue in
the reproductive organs…
2 …travels to the
reproductive organs…
2 …contains a complete set
of genetic information…
3 …where it is transferred
to the gametes.
3 …that is transferred
directly to the gametes.
Sperm
Sperm
Zygote
Egg
Zygote
Egg
1.11 Preformationists in the seventeenth and eightee
centuries believed that sperm or eggs contained fully
humans (the homunculus). Shown here is a drawing of a
homunculus inside a sperm. [Science VU/Visuals Unlimited.
an early
concept
of homuncu
only1.10
one Pangenesis,
parent—from
the father
if the
compared with the modern
the inheritance,
sperm or from
the mother if it was in the egg
germ-plasm theory.
many observations suggested that offspring poss
ture of traits from both parents, preformationism
a popular concept throughout much of the seven
eighteenth centuries.
Another early notion of heredity was blendi
Table 1.1 Early concepts of heredity
Correct or
Incorrect
Concept
Proposed
Pangenesis
Genetic information
travels from different
parts of the body to
reproductive organs.
Incorrect
Inheritance of
acquired
characteristics
Acquired traits become
incorporated into
hereditary information.
Incorrect
Preformationism
Miniature organism
resides in sex cells,
and all traits are
inherited from
one parent.
Incorrect
Blending
inheritance
Genes blend and mix.
Incorrect
Germ-plasm
theory
All cells contain a
complete set of
genetic information.
Correct
Cell theory
All life is composed
of cells, and cells arise
only from cells.
Correct
Mendelian
inheritance
Traits are inherited
in accord with
defined principles.
Correct
had been worked out. Advances in molecular genetics led
to the first recombinant DNA experiments in 1973, which
touched off another revolution in genetic research. Walter
Gilbert (b. 1932) and Frederick Sanger (b. 1918) developed
methods for sequencing DNA in 1977. The polymerase chain
reaction, a technique for quickly amplifying tiny amounts of
DNA, was developed by Kary Mullis (b. 1944) and others
in 1983. In 1990, gene therapy was used for the first time
to treat human genetic disease in the United States, and the
Human Genome Project was launched. By 1995, the firs
complete DNA sequence of a free-living organism—the bacterium Haemophilus influenzae—was determined, and the
first complete sequence of a eukaryotic organism (yeast) was
reported a year later. A rough draft of the human genome
sequence was reported in 2000, with the sequence essentially
completed in 2003, ushering in a new era in genetics (Figure
1.13). Today, the genomes of numerous organisms are being
sequenced, analyzed, and compared. TRY PROBLEMS 22 AND 23
The Future of Genetics
Numerous advances in genetics are being made today, and
genetics remains at the forefront of biological research
New, rapid methods for sequencing DNA are being used to
sequence the genomes of numerous species, from bacteria
to elephants, and the information content of genetics is
increasing at a rapid pace. New details about gene structure
and function are continually expanding our knowledge of
how genetic information is encoded and how it specifies
traits. These findings are redefining what a gene is.
The power of new methods to identify and analyze genes
is illustrated by recent genetic studies of heart attacks in
Caracteres estudiados por Mendel
2.1 MENDEL’S LAWS O
2.1 MENDEL’S LAWS OF INHERITA
CHARACTER
VARIANTS
CHARACTER
CHARACTER
VARIANTS
VARIANTS
CHARACTER
VARIANTS
Seed color
Yellow
Seed color
Height
Yellow
Height
Green
Green
Seed shape
Tall
Tall
Dwarf
Round
Seed shape
Dwarf
Wrinkled
Round
Wrinkled
Green Green
Yellow
Pod color
Pod color
FlowerFlower
colorcolor
Purple
Purple
Yellow
White
White
PodPod
shape
shape
SmoothSmooth
Constricted
Constricted
Flower position
Flower position
Axial
Axial
Terminal
Terminal
F I G U R E 2 . 4 An illustration of the seven characters that Mend
FIGU
Rfound
E 2 .as4twoAnvariants
illustration
the seven
characters
character
was
that wereofdecisively
different
from e
character was found as two variants that were decisively diffe
EXPERIMENT 2A
EXPERIMENT 2A
h of a pollen tube. This enables sperm cells
and migrate toward an ovule. Fertilization
m enters the micropyle, an opening in the
s with an egg cell. The term gamete is used
reproductive cells that can unite to form a
emphasized, however, that the process that
animals is quite different from the way that
ed in plants and fungi. These processes are
detail in Chapter 3.
riments, Mendel wanted to carry out selfmeans that the pollen and egg are derived
In peas, a modified petal known as the keel
ve structures of the plant. Because of this covurally reproduce by self-fertilization. Usually,
en before the flower opens. In other experidel wanted to make crosses between different
accomplish this goal? Fortunately, pea plants
e flowers that are easy to manipulate, making
osses between two particular plants and study
process, known as cross-fertilization, requires
one plant be placed on the stigma of another
e is shown in Figure 2.3. Mendel was able to
flowers and remove the anthers before they
erefore, these flowers could not self-fertilize.
n pollen from another plant by gently touchs with a paintbrush. Mendel applied this polhe flower that already had its anthers removed.
ble to cross-fertilize his pea plants and thereby
brid he wanted.
El cruce
monohíbrido
White
Remove anthers
from purple flower.
Anthers
Parental
generation
Purple
Transfer pollen
from anthers of
white flower to
the stigma of a
purple flower.
Cross-pollinated flower
produces seeds.
Plant the seeds.
Firstgeneration
offspring
FIGURE 2.3 How Mendel cross-fertilized two different pea
pe results from a genotype developing within a spement. The alleles of the genotype, not the phenotype,
PT CHECK 2
P generation Homozygous Homozygous
round seeds wrinkled seeds
!
mong the following terms: locus, allele, genotype, and
ohybrid Crosses Reveal
ciple of Segregation and the
of Dominance
ed with 34 varieties of peas and spent 2 years
se varieties that he would use in his experierified that each variety was pure-breeding
for each of the traits that he chose to study)
he plants for two generations and confirming
ring were the same as their parents. He then
number of crosses between the different varih peas are normally self-fertilizing (each plant
tself), Mendel conducted crosses between difby opening the buds before the anthers (male
ere fully developed, removing the anthers, and
the stigma (female sex organs) with pollen
nt plant’s anthers (Figure 3.4).
began by studying monohybrid crosses—those
nts that differed in a single characteristic. In one
Mendel crossed a pure-breeding (homozygous)
round seeds with one that was pure-breeding
seeds (see Figure 3.4). This first generation of a
(parental) generation.
5 Mendel crossed
two homozygous
varieties of peas.
Cross
F1 generation
!
Selffertilize
6 All the F1 seeds were
round. Mendel allowed
plants grown from
these seeds to selffertilize.
Results
F2 generation
Fraction of
progeny seeds 7
5474 round seeds
3/4 round
1850 wrinkled seeds
1/4 wrinkled
3/ of F
4
2
seeds
were round
and 1/4 were
wrinkled, a
3 !" 1 ratio.
Conclusion: The traits of the parent plants do not blend.
Although F1 plants display the phenotype of one parent,
both traits are passed to F2 progeny in a 3 !" 1 ratio.
3.4 Mendel conducted monohybrid crosses.
primera ley de
Mendel:
‘segregación
igualitaria’
•
genes por
parejas
•
segregan 1:1
en gametos
•
herencia
particulada
such
cted
mete)
ecipkled
were
eeds
wing
that
ertilnerad in
and
ticed
nstihe F2
cted
that
ined
two
n an
splay
netic
pheound
f the
ctors
uded
ctors
ndel
; the
d the
ation
n the
(b)
F1 generation
•Prueba de descendencia
• Cruzamiento prueba
• Cruzamiento recíproco
Round seeds
3 Gametes fused to
produce heterozygous
F1 plants that had
round seeds because
round is dominant
over wrinkled.
Rr
Gamete formation
R r
4 Mendel self-fertilized
the F1 to produce
the F2,…
R r
Gametes
Self–fertilization
(c)
F2 generation
Round
Round
Wrinkled
3/4 round
1/4 wrinkled
5 …which appeared
in a 3 !" 1 ratio of
round to wrinkled.
1/4 Rr
1/4 RR
1/4 rR
1/4 rr
Gamete formation
Gametes R
6 Mendel also selffertilized the F2,…
R
R
r
r
R
r
r
Self–fertilization
(d)
F3 generation
7 …to produce
F3 seeds.
Round Round
RR
Wrinkled Wrinkled
Round
RR
rr
rr
Rr rR
Homozygous round
peas produced
plants with only
round peas.
Heterozygous plants
produced round and
wrinkled seeds in a
3 !" 1 ratio.
Homozygous
wrinkled peas
produced plants with
only wrinkled peas.
CHAPTER 3
URE 3–1
MENDELIAN GENETICS
Character
Contrasting traits
F1 results
F2 results
F2 ratio
Seed shape
round/wrinkled
all round
5474 round
1850 wrinkled
2.96:1
Seed color
yellow/green
all yellow
6022 yellow
2001 green
3.01:1
Pod shape
full/constricted
all full
882 full
299 constricted
2.95:1
Pod color
green/yellow
all green
428 green
152 yellow
2.82:1
Flower
color
violet/white
all violet
705 violet
224 white
3.15:1
axial/terminal
all axial
651 axial
207 terminal
3.14:1
tall/dwarf
all tall
787 tall
277 dwarf
2.84:1
Flower
position
Stem
height
Seven pairs of contrasting traits and the results of Mendel’s seven monohybrid crosses of the garden pea (Pisum sativum). In each ca
X and Y. These chromosomes differ in size and genetic
ion. Certain genes that are found on the X chromosome
ound on the Y chromosome, and vice versa. The X and Y
ndel’s
success
can be attributed to the seven
Genes
in different homologous chromosomes even
omes1 are
notexist
considered
versions called
at
foralleles.
study of
(see
Figure 3.1). He
heyhe
do chose
have short
regions
homology.
ristics
display
range of chromosomes
variation; that
ure 3.3 that
considers
two ahomologous
ed his
withattention
three different
genes.that
An exist
individual
carrying
ed
on those
in two
2 One allele codes
3 …and a different allele
oed
chromosomes
would be homozygous
for theseed
dominant
forms,
for roundsuch
seeds…as white versus
codes gray
for wrinkled
seeds.
gene
A.
The
individual
would
be
heterozygous,
sus wrinkled seeds, and inflated versusBb, for
nd gene. For the third gene, the individual is homozyAllelec.
R The physical
Allelelocation
r
a recessive allele,
of a gene is
del
was
successful
because
he
adopted
an examlocus (plural: loci). As seen in Figure 3.3, for
4 Different alleles occupy the
proach.
Unlike
investigators
ocus of gene
C is many
toward earlier
one endsame
of locus
this on
chromosome,
homologous
chromosomes.
the
B iscrosses,
more in the
middle.formued locus
the rof
esugene
lts of
Mendel
based
onAthis
initial
and then
◗ 3.2
each
locus,observations
a diploid organism
possesses
two alleles located on different homologous
ional
crosses to test his hypotheses. He
chromosomes.
rds of the numbers of progeny possessing
it and
ratios
of
different
locithe
(location)
where computed
the shape of seeds
isGene
determined.
This
locus might be
ose occupied
attention
toallele
detail,
was adept
by an
for round
seeds orat
oneseeing
for wrinkled
seeds.
We patient
will use theand
termthorough,
allele when referring
to a specific
, and
was
conductb term gene toc refer more
version of a gene; we A
will use the
nts generally
for 10 years
before
attempting
to write
to any allele at a locus.
us
The genotype is the set of alleles that an individual
mesorganism possesses. A diploid organism that possesses two
identical alleles is homozygous for that locus. One that posA
B paper (inc
m/pierce
Mendel’s
sesses two different
allelesoriginal
is heterozygous
for the locus.
Another
important
term
is phenotype,
whichccis the
English
translation),
as well
as references,
AA
Bb
Genotype:
manifestationHomozygous
or appearance Heterozygous
of a characteristic.
A phenoHomozygous
mentaries
on
Mendel’s
work
type can referfor
to the
any type of characteristic: physical,
for thephysiological, biochemical,
or behavioral. Thus, therecessive
condition of
dominant
alleleof 50 kg
having roundallele
seeds is a phenotype, a body weight
is a phenotype, and having sickle-cell anemia is a phenone
Mendel’s
crosses
and
book,
the of
term
charthe
actericonclusions
stichromosomes.
c or character refers
E 3type.
. 3 InA this
comparison
homologous
to a general feature such as eye color; the term trait or phe-
nology
the tree’s3.1
genotype
still imposes of
some
limits on its genetic
height:
Table
Summary
important
terms
an oak tree will never grow to be 300 m tall no matter how
much sunlight, water, and fertilizer are provided. Thus, even
Term
Definition
the height of an oak tree is
determined to some degree by
genes. For many characteristics, both genes and environGene
A genetic
factordifferences.
(region of DNA)
ment are important in determining
phenotypic
An obvious but important
only the genothatconcept
helpsis that
determine
a
type is inherited. Although the phenotype is determined, at
characteristic
least to some extent, by genotype,
organisms do not transmit
their phenotypes to the next
generation.
beAllele
One
of twoThe
ordistinction
more alternate
tween genotype and phenotype is one of the most important
forms
a gene
principles of modern genetics.
Theofnext
section describes
Mendel’s careful observation of phenotypes through several
Locus
Specific place on a chromosome
generations of breeding experiments. These experiments aloccupied
by ofantheallele
lowed him to deduce not only
the genotypes
individual
plants, but also the rules governing their inheritance.
Genotype
Concepts
Heterozygote
Set of alleles that an individual
possesses
An individual possessing two
a locus
Each phenotype results from a genotype
different
alleles
developing within a specific
environment.
The at
genotype, not the phenotype, is inherited.
Homozygote
An individual possessing two of
the same alleles at a locus
Monohybrid
Crosses or manifestation
Phenotype or
The appearance
Mendel started with 34 varieties
of peas and spent 2 years
trait
of a character
selecting those varieties that he would use in his experi-
Character
or
An variety
attribute
or feature
ments. He verified
that each
was genetically
pure
(homozygous for each of the traits that he chose to study)
characteristic
by growing the plants for two generations and confirming
that all offspring were the same as their parents. He then
carried out a number of crosses between the different varieties. Although peas are normally self-fertilizing (each plant
crosses with itself), Mendel conducted crosses between dif-
Cruzamiento
dihíbrido
segunda ley de Mendel
Los miembros (alelos) de genes distintos
segregan independientemente durante la
formación de los gametos.
cruzamiento de trihíbridos
E M O N S T R AT E S T H AT M E N D E L’ S P R I N C I P L E S A P P LY TO I N H E R I TA N C E O F M U LT I P L E T R A I T S
w, round individuals
gww
Trihybrid gamete formation
(c) GgWW
GgWW
!
P1
AABBCC
ggww
Gametes
aabbcc
!
ABC
abc
gw
GW
GgWw
F1
gW
AaBbCc
ggWw
ABC
ABc
AbC
Abc
aBC
aBc
abC
abc
Gametes
io
Phenotypic ratio
d
1/2 yellow, round
kled
1/2 green, round
Formation of P1 and F1
gametes in a trihybrid cross.
FIGURE 3–9
51
n= genes
heterocigótico
monohíbrido
1
dihíbrido trihíbrido
2
3
regla
general
n
2
4
8
2n
1/4
1/16
1/64
1/(2n)2
Fenotipos distintos
en F2 (dominancia
completa)
2
4
8
2n
Genotipos distintos
(o fenotipos si no
hay dominancia)
3
9
27
3n
Individuos en F2
(combinaciones de
gametos)
4
16
64
4n
Tipos gametos F1
Homocigóticos
recesivos en F2
Aa x Aa
(A+a)(A+a) = (A+a)2 = AA + 2Aa + aa
AaBb x AaBb
(A+a)(B+b) x (A+a)(B+b) = (A+a)2 (B+b)2
9 AB
3 Ab
3 aB
1 ab
genotipos: (AA + 2Aa + aa)(BB + 2Bb + bb)
fenotipos: (3A + 1a)(3B + 1b)
AaBbDd x AaBbDd
27 ABD
9 ABd
9 AbD
9 aBD
3
3
3
1
Abd
aBd
abD
abd
(A+a)2 (B+b)2 (D+d)2
genotipos: (AA + 2Aa + aa)(BB + 2Bb + bb)(DD +
2Dd + dd)
fenotipos: (3A + 1a)(3B + 1b)(3D + 1d)
Combinaciones de polihíbridos
[(A+a)2]n = (AA + 2Aa + aa)n = (d + 2h + r)n
Nº de genotipos (diferentes) posibles
Probabilidad de un genotipo dado
n!
d! h! r!
1d 2h 1r / 4n
Nº de fenotipos (diferentes) posibles
Probabilidad de un fenotipo dado
n!
D! R!
3D 1R / 4n
Probabilidades totales (nº de combinaciones posibles X P de cada una)
genotipo:
n! 2h
d! h! r! 4n
D
n!
3
fenotipo:
D! R! 4n