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Chapter 14
• Mendel and the gene idea
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Drawing from the Deck of Genes
• What genetic principles account for the
transmission of traits from parents to offspring?
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• One possible explanation  “blending”
hypothesis
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• Alternative to the blending model is
“particulate” hypothesis of inheritance: the
gene idea
– Parents pass on discrete heritable units, genes
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• Gregor Mendel
– Particulate mechanism of inheritance,
experimented with garden peas
Figure 14.1
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• Mendel discovered the basic principles of
heredity by breeding garden peas in carefully
planned experiments
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Used a quantitative approach
• (from Christian Doppler)
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• Mendel chose to work with peas because:
– Available in many varieties
– He could strictly control mating
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• Crossing pea plants
1
APPLICATION By crossing (mating) two true-breeding
varieties of an organism, scientists can study patterns of
inheritance. In this example, Mendel crossed pea plants
that varied in flower color.
TECHNIQUE
Removed stamens
from purple flower
2 Transferred sperm-
bearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
3
Pollinated carpel
matured into
pod
Stamens
Carpel (male)
(female)
4 Planted seeds
from pod
TECHNIQUE
RESULTS
When pollen from a white flower fertilizes
eggs of a purple flower, the first-generation hybrids all have purple
flowers. The result is the same for the reciprocal cross, the transfer
of pollen from purple flowers to white flowers.
Figure 14.2
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5 Examined
First
generation
offspring
(F1)
offspring:
all purple
flowers
• Genetic vocabulary
– Character: a heritable feature, e.g. flower color
– Trait: a variant of a character, e.g. purple or
white flowers
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• Mendel chose to track
– Characters that varied in an “either-or” manner
– Varieties that were “true-breeding”
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• Mendel mated two contrasting, true-breeding
varieties (hybridization)
• The true-breeding parents called P generation
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• Hybrid offspring
–
F1 generation
• When F1 individuals self-pollinate
– F2 generation produced
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The Law of Segregation
• Crossed contrasting, true-breeding white and
purple flowered pea plants
– All offspring were purple
• When F1 plants crossed
– Many offspring plants were purple, but some
were white
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• Mendel discovered
– A ratio of about three to one, purple to white flowers,
in the F2 generation
EXPERIMENT True-breeding purple-flowered pea plants and
white-flowered pea plants were crossed (symbolized by ). The
resulting F1 hybrids were allowed to self-pollinate or were crosspollinated with other F1 hybrids. Flower color was then observed
in the F2 generation.
P Generation
(true-breeding
parents)

Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had
purple flowers
RESULTS Both purple-flowered plants and whiteflowered plants appeared in the F2 generation. In Mendel’s
experiment, 705 plants had purple flowers, and 224 had white
flowers, a ratio of about 3 purple : 1 white.
Figure 14.3
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F2 Generation
• Mendel reasoned that
– In the F1, only the purple flower factor was
affecting flower color in these hybrids
– Purple flower color was dominant, and white
flower color was recessive
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• Mendel observed same pattern in other pea
plant characters
Table 14.1
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• Mendel’s hypothesis:
– Explains the 3:1 inheritance pattern in F2
4 concepts make up model
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
• First, alternative versions of genes
– Variations in inherited characters, called
alleles
Allele for purple flowers
Locus for flower-color gene
Figure 14.4
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Allele for white flowers
Homologous
pair of
chromosomes
• Second, for each character
– Organism inherits two alleles, one f/ each
parent
– A genetic locus is actually represented twice
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• Third, if the two alleles at a locus differ
– One, the dominant allele*, determines the
organism’s appearance
– The other, the recessive allele*, has no
noticeable effect on appearance
* The expression of the allele is dominant or
recessive not the allele itself
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• Fourth, the law of segregation
– The two alleles for a heritable character
separate (segregate) during gamete formation
different gametes
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Mendel’s law of segregation, probability and the
Punnett square
Each true-breeding plant of the
parental generation has identical
alleles, PP or pp.
Gametes (circles) each contain only
one allele for the flower-color gene.
In this case, every gamete produced
by one parent has the same allele.
Union of the parental gametes
produces F1 hybrids having a Pp
combination. Because the purpleflower allele is dominant, all
these hybrids have purple flowers.
When the hybrid plants produce
gametes, the two alleles segregate,
half the gametes receiving the P
allele and the other half the p allele.
This box, a Punnett square, shows
all possible combinations of alleles
in offspring that result from an
F1  F1 (Pp  Pp) cross. Each square
represents an equally probable product
of fertilization. For example, the bottom
left box shows the genetic combination
resulting from a p egg fertilized by
a P sperm.
P Generation

Appearance:Purple flowers White flowers
Genetic makeup: PP
pp
Gametes:
p
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 P
F1 sperm
P
p
PP
Pp
F2 Generation
P
F1 eggs
p
pp
Pp
Figure 14.5
Random combination of the gametes
results in the 3:1 ratio that Mendel
observed in the F2 generation.
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2 p
3
:1
• If homozygous f/ a particular gene
– Pair of identical alleles
– True-breeding
• If heterozygous f/ a particular gene
– Pair of alleles that are different
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• Phenotype
– Physical appearance
• Genotype
– Genetic makeup
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• Phenotype v. genotype
Phenotype
Purple
3
Purple
Genotype
PP
(homozygous)
1
Pp
(heterozygous)
2
Pp
(heterozygous)
Purple
1
Figure 14.6
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
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1
Testcross
• In pea plants with purple flowers, genotype is
not obvious
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• Testcross
APPLICATION An organism that exhibits a dominant trait,
such as purple flowers in pea plants, can be either homozygous for
the dominant allele or heterozygous. To determine the organism’s
genotype, geneticists can perform a testcross.

Dominant phenotype,
unknown genotype:
PP or Pp?
TECHNIQUE In a testcross, the individual with the
unknown genotype is crossed with a homozygous individual
expressing the recessive trait (white flowers in this example).
By observing the phenotypes of the offspring resulting from this
cross, we can deduce the genotype of the purple-flowered
parent.
Recessive phenotype,
known genotype:
pp
If PP,
then all offspring
purple:
p
If Pp,
then 2 offspring purple
and 1⁄2 offspring white:
1⁄
p
p
p
Pp
Pp
pp
pp
RESULTS
P
P
Pp
Pp
P
p
Pp
Figure 14.7
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Pp
Law of segregation
• Determined by following a single trait
• F1 were monohybrids, heterozygous for one
character
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The Law of Independent Assortment
• Follow 2 characters at the same time
• Crossing two, true-breeding parents differing in
two characters
dihybrids in the F1 generation, heterozygous
for both characters
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• How are two characters transmitted f/ parents
to offspring?
– As a package?
– Independently?
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• Dihybrid cross
• 4 phenotypes in the F2 generation
P Generation
YYRR
yyrr
Gametes
F1 Generation
YR

yr
YyRr
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm
1⁄
4
Sperm
1⁄
2
F2 Generation
Eggs
1⁄
2 YR
(predicted
offspring)
1⁄
2
YR
YYRR
1⁄
2
1⁄
4
Yr
1⁄
4
yR
1⁄
4
1⁄
4
YR
1⁄
4
Yr
1⁄
4
yR
1⁄
4
yr
YYRR
YYRr
YyRR
YyRr
YYrr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yr
yyrr
YyRr
3⁄
4
1⁄
4
Phenotypic ratio 3:1
9⁄
16
3⁄
16
3⁄
16
yyrr
1⁄
16
Phenotypic ratio 9:3:3:1
315
yr
Eggs
yr
YyRr
YR
108
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101
32
Phenotypic ratio approximately 9:3:3:1
• Law of independent assortment
– Each pair of alleles segregates independently
during gamete formation
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• The multiplication rule
– Probability that two or more independent
events will occur together is the product of
their individual probabilities
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• Probability in a monohybrid cross

Rr
Rr
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1⁄
R
2
R
1⁄
R
2
1⁄
Eggs
⁄2
1
r
R
r
Figure 14.9
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1⁄
R
1⁄
4
R
1⁄
4
r
2
r
r
4
r
1⁄
4
• Rule of addition
– Probability that any one of two or more
exclusive events will occur is sum of individual
probabilities
– e.g. probability of a heterozygote in a Tt xTt
cross ¼ prob. of Tt and ¼ prob. Of tT
 ¼ + ¼ = ½ probability of heterozygote
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• Dihybrid
– Equivalent to two independent monohybrid
crosses occurring simultaneously
• When calculating:
– Multiply individual probabilities
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• Inheritance patterns are often more complex
than predicted by simple Mendelian genetics
• Relationship between genotype and phenotype
is rarely simple
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The Spectrum of Dominance
• Complete dominance
– Phenotypes of the heterozygote and dominant
homozygote are identical
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• Codominance
– 2 dominant alleles affect the phenotype in
separate, distinguishable ways
– e.g. human blood group MN
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• Incomplete dominance
– Phenotype of F1 hybrids between phenotypes of the
two parental varieties
P Generation
Red
CRCR
White
CW CW

Gametes CR
CW
Pink
CRCW
F1 Generation
Gametes
Eggs
F2 Generation
Figure 14.10
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1⁄
2
CR
1⁄
2
Cw
1⁄
2
1⁄
2
CR
1⁄
2
CR
CR 1⁄2 CR
CR CR CR CW
CR CW CW CW
Sperm
• Relation Between Dominance and Phenotype
• Dominant and recessive alleles
– Do not really “interact”
– Lead to synthesis of different proteins that
produce a phenotype
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• Dominant alleles
– Not necessarily more common in populations
than recessive alleles
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Multiple alleles
• e.g. ABO blood group in humans
• More than 2 alleles
Table 14.2
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• Pleiotropy
– Gene has multiple phenotypic effects
– e.g. sickle-cell disease
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• e.g. Epistasis
– Gene at one locus alters the phenotypic
expression of a gene at second locus
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• Example of epistasis

BbCc
BbCc
Sperm
1⁄
BC
4
1⁄
4
bC
1⁄
4
1⁄
Bc
4
bc
Eggs
1⁄
1⁄
4
BC
BBCC
BbCC
BBCc
BbCc
4
bC
BbCC
bbCC
BbCc
bbCc
1⁄
1⁄
4
Bc
BBCc
BbCc
BBcc
4
bc
BbCc
bbCc
Bbcc
9⁄
16
Figure 14.11
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3⁄
16
Bbcc
4⁄
bbcc
16
Polygenic inheritance
• Quantitative variation  polygenic inheritance
– Additive effect of two or more genes,
continuum

AaBbCc
AaBbCc
aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCcAABBCC
20⁄
15⁄
6⁄
Figure 14.12
64
64
64
1⁄
64
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Nature and Nurture: Environmental Impact on
Phenotype
– Phenotype depends on environment &
genotype
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• Norm of reaction
– Phenotypic range of a particular genotype that
is influenced by the environment
Figure 14.13
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• Multifactorial characters
– Influenced by both genetics and environment
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• Phenotype
– Physical appearance, internal anatomy,
physiology, and behavior
– Genotype + unique environmental history
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Pedigree
• Inheritance patterns of particular traits
Ww
ww
Ww ww ww Ww
WW
or
Ww
ww
Ww
Ww
ww
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
FF or Ff
Ff
Ff
Third
generation
(two sisters)
ww
Widow’s peak
Ff
No Widow’s peak
(a) Dominant trait (widow’s peak)
Figure 14.14 A, B
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Attached earlobe
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Free earlobe
(b) Recessive trait (attached earlobe)
• Recessively inherited disorders
– Show up only in individuals homozygous for
the allele
• Carriers
– Heterozygous individuals who carry the
recessive allele but are phenotypically normal
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• Sickle-cell disease
– Caused by the substitution of a single amino
acid in the hemoglobin protein in red blood
cells
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• Matings between relatives
– Increase the probability of the appearance of a
genetic disease
– Called consanguineous matings
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Dominantly inherited disorders
• e.g. achondroplasia
Figure 14.15
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Multifactorial Disorders
• Many human diseases
– Genetic and environment components
– e.g. Heart disease
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Genetic Testing and Counseling
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• Fetal testing
(b) Chorionic villus sampling (CVS)
(a) Amniocentesis
Amniotic
fluid
withdrawn
A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
Fetus
Fetus
Suction tube
Inserted through
cervix
Centrifugation
Placenta
Placenta
Uterus
Chorionic viIIi
Cervix
Fluid
Fetal
cells
Fetal
cells
Biochemical tests can be
Performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Biochemical
tests
Several
weeks
Several
hours
Karyotyping
Figure 14.17 A, B
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Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.
Ultrasound
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