Transcript F 1

Chapter 9
Patterns of Inheritance
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Introduction
 Dogs are one of man’s longest genetic
experiments.
– Over thousands of years, humans have chosen and
mated dogs with specific traits.
– The result has been an incredibly diverse array of dogs
with distinct
– body types and
– behavioral traits.
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Wolf
Ancestral
canine
Chinese Shar-Pei
Akita
Siberian Husky
Basenji
Alaskan Malamute
Afghan hound
Saluki
Rottweiler
Sheepdog
Retriever
Figure 9.0_1
Chapter 9: Big Ideas
Mendel’s Laws
The Chromosomal Basis
of Inheritance
Variations on
Mendel’s Laws
Sex Chromosomes and
Sex-Linked Genes
MENDEL’S LAWS
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9.2 Experimental genetics began in an abbey
garden
 Heredity is the transmission of traits from one
generation to the next.
 Genetics is the scientific study of heredity.
 Gregor Mendel
– began the field of genetics in the 1860s,
– deduced the principles of genetics by breeding garden
peas, and
– relied upon a background of mathematics, physics, and
chemistry.
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9.2 Experimental genetics began in an abbey
garden
 In 1866, Gregor Mendel
– correctly argued that parents pass on to their offspring
discrete “heritable factors” and
– stressed that the heritable factors (today called genes),
retain their individuality generation after generation.
 A heritable feature that varies among individuals,
such as flower color, is called a character.
 Each variant for a character, such as purple or white
flowers, is a trait.
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Figure 9.2A
Gregor Mendel
9.2 Experimental genetics began in an abbey
garden
 True-breeding varieties result when self-fertilization
produces offspring all identical to the parent.
 The offspring of two different varieties are hybrids.
 The cross-fertilization is a hybridization, or genetic
cross.
 True-breeding parental plants are the P generation.
 Hybrid offspring are the F1 generation.
 A cross of F1 plants produces an F2 generation.
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Figure 9.2D
Traits
Character
Dominant
Recessive
Purple
White
Axial
Terminal
Yellow
Green
Round
Wrinkled
Inflated
Constricted
Green
Yellow
Tall
Dwarf
Flower color
Flower position
Seed color
Seed shape
Pod shape
Pod color
Stem length
Figure 9.2D_1
Traits
Character
Dominant
Recessive
Purple
White
Axial
Terminal
Yellow
Green
Round
Wrinkled
Flower color
Flower position
Seed color
Seed shape
Figure 9.2D_2
Traits
Character
Dominant
Recessive
Inflated
Constricted
Green
Yellow
Tall
Dwarf
Pod shape
Pod color
Stem length
Figure 9.2B
Petal
Carpel
Stamen
Figure 9.2C_s1
White
1 Removal of
stamens
Stamens
Carpel
Parents
(P)
2 Transfer
Purple of pollen
Figure 9.2C_s2
White
1 Removal of
stamens
Stamens
Carpel
Parents
(P)
2 Transfer
Purple of pollen
3 Carpel matures
into pea pod
Figure 9.2C_s3
White
1 Removal of
stamens
Stamens
Carpel
Parents
(P)
2 Transfer
Purple of pollen
3 Carpel matures
into pea pod
4 Seeds from
pod planted
Offspring
(F1)
9.3 Mendel’s law of segregation describes the
inheritance of a single character
Monohybrid cross
(single factor cross)
 A cross between two individuals differing in a
single character
 Mendel performed a monohybrid cross between a
plant with purple flowers and a plant with white
flowers.
– The F1 generation produced all plants with purple
flowers.
– A cross of F1 plants with each other produced an F2
generation with ¾ purple and ¼ white flowers.
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Figure 9.3A_s1
The Experiment
P generation
(true-breeding
parents)

Purple
flowers
White
flowers
Figure 9.3A_s2
The Experiment
P generation
(true-breeding
parents)

Purple
flowers
F1 generation
White
flowers
All plants have
purple flowers
Figure 9.3A_s3
The Experiment
P generation
(true-breeding
parents)

Purple
flowers
F1 generation
White
flowers
All plants have
purple flowers
Fertilization
among F1 plants
(F1  F1)
F2 generation
3
4
1 of plants
of plants
4
have purple flowers have white flowers
9.3 Mendel’s law of segregation describes the
inheritance of a single character
 Mendel needed to explain why
– white color seemed to disappear in the F1 generation
and
– white color reappeared in one quarter of the F2
offspring.
 Mendel observed the same patterns of inheritance
for six other pea plant characters.
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9.3 Mendel’s law of segregation describes the
inheritance of a single character
 Mendel developed four hypotheses, described below
using modern terminology.
1. Alleles are alternative versions of genes that
account for variations in inherited characters.
2. For each characteristic, an organism inherits two
alleles, one from each parent. The alleles can be
the same or different.
– A homozygous genotype has identical alleles.
– A heterozygous genotype has two different alleles.
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9.3 Mendel’s law of segregation describes the
inheritance of a single character
3. If the alleles of an inherited pair differ, then one
determines the organism’s appearance and is
called the dominant allele.
– The other has no noticeable effect on the
organism’s appearance and is called the
recessive allele.
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Genotype vs. Phenotype
 The phenotype is the appearance or
expression of a trait
– Eg: Purple flowers, yellow seeds, short legs (of dogs)
 The genotype is the genetic make up of
alleles located on chromosomes.
– Eg: PP, Pp, or pp,
 The same phenotype may be determined by
more than one genotype

Eg: Purple flowers can have genotype of PP or Pp
9.3 Mendel’s law of segregation describes the
inheritance of a single character
4. The law of segregation.
Allele pairs separate (segregate) from each other
during the production of gametes.
Therefore, a sperm or an egg carries only one
allele for each inherited character
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Figure 9.3B_s1
The Explanation
P generation
Genetic makeup (alleles)
White flowers
Purple flowers
PP
pp
Gametes
All P
All p
Figure 9.3B_s2
The Explanation
P generation
Genetic makeup (alleles)
White flowers
Purple flowers
PP
pp
Gametes
All P
All p
F1 generation
(hybrids)
All Pp
Gametes
1
2
P
1
2
p
Figure 9.3B_s3
The Explanation
P generation
Genetic makeup (alleles)
White flowers
Purple flowers
PP
pp
Gametes
All P
All p
F1 generation
(hybrids)
All Pp
Gametes
1
2
P
Alleles
segregate
1
2
p
Fertilization
Sperm from F1 plant
F2 generation
P
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
p
P
PP
Pp
Eggs
from F1
plant
p
Pp
pp
Figure 9.3B_4
F2 generation
Phenotypic ratio
3 purple : 1 white
Sperm from F1 plant
P
Eggs
from F1
plant
Genotypic ratio
p
1 PP : 2 Pp : 1 pp
P
p
PP
Pp
Pp
pp
9.4 Homologous chromosomes bear the alleles
for each character
 A locus (plural, loci) is the specific location of a
gene along a chromosome.
 For a pair of homologous chromosomes, alleles of
a gene reside at the same locus.
– Homozygous individuals have the same allele on both
homologues.
– Heterozygous individuals have a different allele on
each homologue.
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Figure 9.4
Gene loci
P
a
B
P
a
b
Dominant
allele
Homologous
chromosomes
Genotype: PP
Homozygous
for the
dominant
allele
aa
Homozygous
for the
recessive
allele
Recessive
allele
Bb
Heterozygous,
with one dominant
and one recessive
allele
Mendel’s Monohybrid cross (summary)
– Describes the inheritance pattern of a single
character
– F2 phenotype ratio is 3:1
– Law of segregation is applied to two alleles of
a single gene (eg: P & p). This means that the
two alleles of a single gene are separated, and
go to different gametes during meiosis.
Copyright © 2009 Pearson Education, Inc.
9.5 The law of independent assortment is
revealed by tracking two characters at once
 A dihybrid cross is a mating of parental varieties
that differ in two characters.
(two-factor cross)
 Mendel performed the following dihybrid cross:
– Seed color and seed shape
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9.5 The law of independent assortment is
revealed by tracking two characters at once
 Mendel performed the following dihybrid cross with
the following results:
– P generation: round yellow seeds  wrinkled green seeds
– F1 generation: all plants with round yellow seeds
– F2 generation:
– 9/16 had round yellow seeds
– 3/16 had wrinkled yellow seeds
– 3/16 had round green seeds
– 1/16 had wrinkled green seeds
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Figure 9.5A_1
Dihybrid cross of Seed color and Seed shape
Seed color: YY or Yy –Yellow
yy- green
Seed shape: RRor Rr –Round
rr- wrinkled
rryy
P generation RRYY
Gametes RY
F1 generation

ry
RrYy
Figure 9.5A_3
RrYy
F1 generation
Sperm
1
4
1
4
RY
1
4
rY
RY
RRYY
RrYY
Eggs
1
4
1
4
1
4
rY
RrYY
rrYY
1
4
Ry
RRYy
RrYy
1
4
ry
RrYy
rrYy
Ry
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
9
16
Yellow
round
3
16
Green
round
3
16
Yellow
wrinkled
1
16
Green
wrinkled
The hypothesis of independent assortment
Actual results; hypothesis supported
9.5 The law of independent assortment is
revealed by tracking two characters at once
 Mendel needed to explain why the F2 offspring
– had new nonparental combinations of traits and
– a 9:3:3:1 phenotypic ratio.
 Mendel
– suggested that the inheritance of one character has no
effect on the inheritance of another,
– suggested that the dihybrid cross is the equivalent to two
monohybrid crosses, and
– called this the law of independent assortment.
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9.5 The law of independent assortment is
revealed by tracking two characters at once
 Law of independent assortment
– Each pair of alleles segregates independently of the
other pairs of alleles during gamete formation
– For genotype RrYy, four gamete types are possible:
RY, Ry, rY, and ry
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THE CHROMOSOMAL BASIS
OF INHERITANCE
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9.16 Chromosome behavior accounts for
Mendel’s laws
 Mendel’s laws correlate with chromosome
separation in meiosis.
 The two alleles for a single trait are located on
homologous chromosomes
 The law of segregation depends on separation of
homologous chromosomes in anaphase I.
 The law of independent assortment depends on
alternative orientations of chromosomes in
metaphase I.
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Figure 9.16_s1
F1 generation
R
r
y
All yellow round seeds
(RrYy)
Y
R
Y
r
y
Metaphase I
of meiosis
r
R
Y
y
Figure 9.16_s2
F1 generation
R
r
y
All yellow round seeds
(RrYy)
Y
r
R
Y
R
y
Metaphase I
of meiosis
r
R
Y
y
r
r
R
Y
y
Anaphase I
Y
y
Metaphase II
R
r
r
R
Y
y
Y
y
Figure 9.16_s3
F1 generation
R
r
All yellow round seeds
(RrYy)
y
Y
r
R
Y
R
y
Metaphase I
of meiosis
r
R
Y
y
r
r
R
Y
y
Anaphase I
Y
y
Metaphase II
R
r
r
R
Y
y
Y
y
Gametes
Y
Y
R
R
1
4
RY
y
y
r
r
1
4
Y
Y
r
r
ry
F2 generation 9
Fertilization
:3
:3
:1
1
4
rY
y
y
R
R
1
4
Ry
Figure 9.16_4
Sperm
1
1
1
1
4 RY 4 rY 4 Ry 4 ry
1
4 RY RRYY RrYY RRYy RrYy
1
4 rY
Eggs
1 Ry
4
1 ry
4
RrYY rrYY RrYy rrYy
RRYy RrYy RRyy Rryy
RrYy rrYy
Rryy rryy
9
16
Yellow
round
3
16
Green
round
3
16
Yellow
wrinkled
1
16
Green
wrinkled
Mendel’s Dihybrid cross (summary)
– Describes the inheritance pattern of two
characters at the same time
– F2 phenotype ratio is 9:3:3:1
– Law of independent assortment is applied to
two alleles of two genes (eg: Y, y and R, r).
This means any one of the alleles from color
gene (Y or y) can combine with any one allele
of the seed shape gene (R or r).
Copyright © 2009 Pearson Education, Inc.
9.5 The law of independent assortment is
revealed by tracking two characters at once
 The following figure demonstrates the law of
independent assortment as it applies to two
characters in Labrador retrievers:
– black versus chocolate color,
– normal vision versus progressive retinal atrophy.
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Figure 9.5B_1
Independent assortment of two genes in the Labrador retriever
Blind
Phenotypes
Genotypes
Black coat,
normal vision
B_N_
Black coat,
blind (PRA)
B_nn
Blind
Phenotypes
Genotypes
Chocolate coat,
normal vision
bbN_
Chocolate coat,
blind (PRA)
bbnn
Figure 9.5B_2
Mating of double heterozygotes (black coat, normal vision)
BbNn 
BbNn
Blind
Phenotypic
ratio of the
offspring
9
Black coat,
normal vision
Blind
1
3
3
Black coat, Chocolate coat, Chocolate coat,
blind (PRA)
blind (PRA) normal vision
9.6 Geneticists can use the testcross to determine
unknown genotypes
 A testcross is the mating between an individual of
unknown genotype and a homozygous recessive
individual.
 A testcross can show whether the unknown
genotype includes a recessive allele.
 Mendel used testcrosses to verify that he had truebreeding genotypes.
© 2012 Pearson Education, Inc.
Figure 9.6
What is the genotype of the black dog?

Testcross
Genotypes
B_?
bb
Two possibilities for the black dog:
BB
Gametes
B
b
Offspring
Bb
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
9.7 Mendel’s laws reflect the rules of probability
 Using his strong background in mathematics,
Mendel knew that the rules of mathematical
probability affected
– the segregation of allele pairs during gamete formation
and
– the re-forming of pairs at fertilization.
 The probability scale ranges from 0 to 1. An event
that is
– certain has a probability of 1 and
– certain not to occur has a probability of 0.
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9.7 Mendel’s laws reflect the rules of probability
 The probability of a specific event is the number of
ways that event can occur out of the total possible
outcomes.
 Determining the probability of two independent
events uses the rule of multiplication, in which
the probability is the product of the probabilities for
each event.
 The probability that an event can occur in two or
more alternative ways is the sum of the separate
probabilities, called the rule of addition.
© 2012 Pearson Education, Inc.
Figure 9.7
F1 genotypes
Bb male
Bb female
Formation
of eggs
Formation
of sperm
1
2
1
2
B
b
Sperm
1
1
(2  2 )
1
2
F2 genotypes
B
b
b
1
4
B
1
4
b
B
1
4
Eggs
1
2
B
B
b
b
1
4
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
 A pedigree
– Shows the inheritance of a trait in a family through
multiple generations
– Demonstrates dominant or recessive inheritance
– Can also be used to deduce genotypes of family
members
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Figure 9.8A
Dominant Traits
Recessive Traits
Examples of single-gene
inherited traits in humans
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
 A pedigree
– shows the inheritance of a trait in a family
through multiple generations,
– demonstrates dominant or recessive
inheritance, and
– can also be used to deduce genotypes of
family members.
© 2012 Pearson Education, Inc.
Figure 9.8B
Inheritance of attached versus free earlobes
First generation
(grandparents)
Second generation
(parents, aunts,
FF
and uncles)
or
Ff
Third generation
(two sisters)
Female
Male
Attached
Free
Ff
ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
 These disorders are inherited in two ways
 Dominant inheritance
 Recessive inheritance
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9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
 Recessive inheritance in which
– two recessive alleles are needed to show disease,
– heterozygous parents are carriers of the disease-causing
allele, and
– the probability of inheritance increases with inbreeding,
mating between close relatives.
– Eg: Albinism, Cystic Fibrosis, Tay-Sach’s,
– inherited form of deafness
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Figure 9.9A
Normal
Dd
Parents
D
Inheritance of deafness,
a recessive trait
D
Offspring
Normal
Dd

Sperm
d
DD
Normal
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Eggs
d
9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
 The most common fatal genetic disease in the
United States is cystic fibrosis (CF), resulting in
excessive thick mucus secretions.
 The CF allele is
– recessive and
– carried by about 1 in 31 Americans.
– CF is mostly observed in Caucasians.
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Phenotype = albinism
Genotype = aa
9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
 Dominant inheritance in which
– one dominant allele is needed to show disease and
– dominant lethal alleles are usually eliminated from the
population.
– Eg: Achondroplasia, resulting in dwarfism, and
– Huntington’s disease, a degenerative disorder of the
nervous system.
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Figure 9.9B
Dr. Michael C. Ain, a specialist in the repair of bone defects caused
by achondroplasia and related disorders
Q: What is his genotype for the achondroplasia gene?
Table 9.9
VARIATIONS ON
MENDEL’S LAWS
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VARIATIONS ON MENDEL’S
LAWS
 Incomplete dominance
 Multiple alleles
 Codominance
 Pleiotropy
 Polygenic inheritance
 Environmental influence
 Linked genes
 Genes located on sex chromosomes
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9.11 Incomplete dominance results in
intermediate phenotypes
 Mendel’s pea crosses always looked like one of the
parental varieties, called complete dominance.
 For some characters, the appearance of F1 hybrids
falls between the phenotypes of the two parental
varieties. This is called incomplete dominance, in
which
– neither allele is dominant over the other and
– expression of both alleles occurs.
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Figure 9.11A
P generation

Red
RR
White
rr
Gametes R
r
F1 generation
Pink hybrid
Rr
Gametes
1
2 R
1
2 r
Sperm
1
1
R
2
2 r
F2 generation
1 R
2
RR
rR
1 r
2
Rr
rr
Eggs
9.11 Incomplete dominance results in
intermediate phenotypes
 Incomplete dominance does not support the
blending hypothesis because the original parental
phenotypes reappear in the F2 generation.
 One example of incomplete dominance in humans
is hypercholesterolemia, in which
– dangerously high levels of cholesterol occur in the blood
and
– heterozygotes have intermediately high cholesterol
levels.
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Figure 9.11B
Incomplete dominance in human hypercholesterolemia
Genotypes
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
Phenotypes
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
9.12 Many genes have more than two alleles in
the population
 Multiple alleles
– More than two alleles are found in the population
– A diploid individual can carry any two of these alleles
– The ABO blood group has three alleles, leading to four
phenotypes: type A, type B, type AB, and type O blood
Copyright © 2009 Pearson Education, Inc.
9.12 Many genes have more than two alleles in
the population
 The A and B alleles are both expressed in
heterozygous individuals, a condition known as
codominance.
 In codominance,
– neither allele is dominant over the other and
– expression of both alleles is observed as a distinct
phenotype in the heterozygous individual.
– AB blood type is an example of codominance.
© 2012 Pearson Education, Inc.
Figure 9.12
Blood
Group
(Phenotype)
Genotypes
Carbohydrates Present
on Red Blood Cells
Carbohydrate A
A
IAIA
or
I Ai
Carbohydrate B
B
IBIB
or
IBi
AB
IAIB
Antibodies
Present
in Blood
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left
O
A
B
AB
Anti-B
Anti-A
Carbohydrate A
and
Carbohydrate B
None
Anti-A
O
ii
Neither
Anti-B
No reaction
Clumping reaction
Figure 9.12_1
Blood
Group
(Phenotype)
Genotypes
Carbohydrates Present
on Red Blood Cells
A
IA IA
or
IA i
Carbohydrate A
Carbohydrate B
B
IB IB
or
IB i
AB
IA IB
O
ii
Carbohydrate A
and
Carbohydrate B
Neither
9.13 A single gene may affect many phenotypic
characters
 Pleiotropy occurs when one gene influences many
characteristics.
 Sickle-cell disease is a human example of pleiotropy.
This disease
– affects the type of hemoglobin produced and the shape of
red blood cells and
– causes anemia and organ damage.
– Sickle-cell and nonsickle alleles are codominant.
– Carriers of sickle-cell disease are resistant to malaria.
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Figure 9.13A
Figure 9.UN03
Single
gene
Pleiotropy
Multiple characters
Figure 9.13B
An individual homozygous for the sickle-cell allele
Produces sickle-cell (abnormal) hemoglobin
The abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickled cell
The multiple effects of sickled cells
Damage to organs
Other effects
Kidney failure
Heart failure
Spleen damage
Brain damage (impaired
mental function,
paralysis)
Pain and fever
Joint problems
Physical weakness
Anemia
Pneumonia and other
infections
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
9.14 A single character may be influenced by
many genes
Polygenic inheritance
 A single phenotypic character results from the
additive effects of two or more genes.
 Human skin color is an example of polygenic
inheritance.
© 2012 Pearson Education, Inc.
Figure 9.UN04
Multiple
genes
Polygenic
inheritance
Single characters
(such as skin color)
Figure 9.14
P generation

aabbcc
AABBCC
(very light) (very dark)
F1 generation

AaBbCc AaBbCc
Sperm
1
8
F2 generation
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
Fraction of population
Eggs
1
8
1
8
1
8
1
8
1
8
1
64
6
64
15
64
20
64
15
64
6
64
1
64
Skin color
Figure 9.14_1
P generation

aabbcc
AABBCC
(very light) (very dark)
F1 generation

AaBbCc
AaBbCc
Figure 9.14_2
Sperm
1
8
F2 generation
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
Eggs
1
8
1
8
1
8
1
8
1
8
1
64
6
64
15
64
20
64
15
64
6
64
1
64
Figure 9.14_3
Fraction of population
20
64
15
64
6
64
1
64
Skin color
9.15 The environment affects many characters
 Environmental influence
 Many characters result from a combination of heredity and
the environment. For example,
– skin color is affected by exposure to sunlight,
– susceptibility to diseases, such as cancer, has
hereditary and environmental components, and
– identical twins show some differences.
 Only genetic influences are inherited.
© 2012 Pearson Education, Inc.
Figure 9.15A
http://www.guardian.co.uk/lifeandstyle/2011/sep/24
/twins-black-white?fb=optOut
Figure 9.15B
Varying phenotypes due to environmental factors in
genetically identical twins
9.17 Genes on the same chromosome tend to be
inherited together
 Linked Genes
– Are located close together on the same chromosome
– Tend to be inherited together
Copyright © 2009 Pearson Education, Inc.
Figure 9.19A
Section of chromosome carrying linked genes
g
c
l
17%
9%
9.5%
Recombination
frequencies
Figure 9.19B
Mutant phenotypes
Short
aristae
Black
body
(g)
Cinnabar Vestigial
wings
eyes
(l)
(c)
Red
Long aristae Gray
Normal
eyes
(appendages body
wings
(C)
on head)
(G)
(L)
Wild-type phenotypes
Brown
eyes
Red
eyes
SEX CHROMOSOMES AND
SEX-LINKED GENES
© 2012 Pearson Education, Inc.
9.20 Chromosomes determine sex in many
species
 Many animals have a pair of sex chromosomes,
– designated X and Y,
– that determine an individual’s sex.
 In mammals,
– males have XY sex chromosomes,
– females have XX sex chromosomes,
– the Y chromosome has genes for the development of
testes, and
– an absence of the Y allows ovaries to develop.
© 2012 Pearson Education, Inc.
9.21 Sex-linked genes exhibit a unique pattern of
inheritance
 Sex-linked genes are located on either of the sex
chromosomes.
 The X chromosome carries many genes unrelated
to sex.
 The inheritance of white eye color in the fruit fly
illustrates an X-linked recessive trait.
© 2012 Pearson Education, Inc.
Figure 9.20A
X
Y
Figure 9.20B
Male
44

XY
Parents
(diploid)
Gametes
(haploid)
Offspring
(diploid)
22

X
22

Y
Sperm
44

XX
Female
Female
44

XX
22

X
Egg
44

XY
Male
Figure 9.21A
Fruit fly eye color determined by sex-linked gene
Figure 9.21B
Female
Male
XRXR
XrY
Sperm
Eggs XR
Xr
Y
XRXr
XRY
R  red-eye allele
r  white-eye allele
Figure 9.21C
Female
Male
XRXr
XRY
Sperm
Y
xR
XR
XRXR
XRY
XrXR
XrY
Eggs
Xr
R  red-eye allele
r  white-eye allele
Figure 9.21D
Female
Male
XRXr
XrY
Sperm
Xr
Y
XR
XRXr
XRY
Xr
XrXr
XrY
Eggs
R  red-eye allele
r  white-eye allele
9.22 CONNECTION: Human sex-linked
disorders affect mostly males
 Most sex-linked human disorders are
– due to recessive alleles and
– seen mostly in males.
 A male receiving a single X-linked recessive allele
from his mother will have the disorder.
 A female must receive the allele from both parents
to be affected.
© 2012 Pearson Education, Inc.
9.22 CONNECTION: Human sex-linked
disorders affect mostly males
 Recessive and sex-linked human disorders
include
– hemophilia, characterized by excessive bleeding
because hemophiliacs lack one or more of the proteins
required for blood clotting,
– red-green color blindness, a malfunction of lightsensitive cells in the eyes, and
– Duchenne muscular dystrophy, a condition
characterized by a progressive weakening of the
muscles and loss of coordination.
© 2012 Pearson Education, Inc.
Figure 9.22
Hemophilia in the royal family of Russia
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Female Male
Hemophilia
Carrier
Normal
9.23 EVOLUTION CONNECTION: The Y
chromosome provides clues about human
male evolution
 The Y chromosome provides clues about human
male evolution because
– Y chromosomes are passed intact from father to son
and
– mutations in Y chromosomes can reveal data about
recent shared ancestry.
© 2012 Pearson Education, Inc.
Figure 9.23