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Chapter 9
Patterns of Inheritance
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
© 2010 Pearson Education, Inc.
HERITABLE VARIATION AND PATTERNS
OF INHERITANCE
• Heredity is the transmission of traits from one generation to
the next.
• Gregor Mendel
– Worked in the 1860s
– Was the first person to analyze
patterns of inheritance
– Deduced the fundamental
principles of genetics
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In an Abbey Garden
• Mendel studied garden peas because they
– Are easy to grow
– Come in many readily distinguishable varieties
– Are easily manipulated
– Can self-fertilize
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Petal
Stamen
(makes spermproducing
pollen)
Carpel
(produces eggs)
Figure 9.2
• Mendel
– Created true-breeding varieties of plants
– Crossed two different true-breeding varieties
• Hybrids are the offspring of two different true-breeding
varieties.
– The parental plants are the P generation.
– Their hybrid offspring are the F1 generation.
– A cross of the F1 plants forms the F2 generation.
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Removed
stamens
from purple
flower.
White
Stamens
Parents
(P)
Carpel
Purple
Transferred pollen from
stamens of white flower
to carpel of purple
flower.
Figure 9.3-1
White
Removed
stamens
from purple
flower.
Stamens
Parents
(P)
Carpel
Purple
Transferred pollen from
stamens of white flower
to carpel of purple
flower.
Pollinated carpel
matured into pod.
Planted seeds
from pod.
Offspring
(F1)
Figure 9.3-3
Mendel’s Law of Segregation
• Mendel performed many experiments.
• He tracked the inheritance of characters that occur as two
alternative traits.
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• A character is a heritable feature that varies among individuals.
• A trait is a variant of a character.
• Each of the characters Mendel studied occurred in two distinct
forms.
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Dominant Recessive
Flower color
Dominant Recessive
Pod shape
Purple
White
Flower position
Inflated
Constricted
Pod color
Green
Yellow
Tall
Dwarf
Stem length
Axial
Terminal
Yellow
Green
Seed color
Seed shape
Round
Wrinkled
Figure 9.4
P Generation
(true-breading
parents)
Purple flowers
White flowers
Figure 9.5-1
P Generation
(true-breading
parents)
Purple flowers
F1 Generation
White flowers
All plants have
purple flowers
Figure 9.5-2
P Generation
(true-breading
parents)
Purple flowers
F1 Generation
White flowers
All plants have
purple flowers
Fertilization
among F1 plants
(F1  F1)
F2 Generation
3
of plants
4
have purple flowers
1
of plants
4
have white flowers
Figure 9.5-3
• Mendel developed four hypotheses from the monohybrid cross:
1. There are alternative versions of genes, called alleles.
2. For each character, an organism inherits two alleles, one from each parent.
– An organism is homozygous for that gene if both alleles are
identical.
– An organism is heterozygous for that gene if the alleles are different.
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3. If two alleles of an inherited pair differ
– The allele that determines the organism’s appearance is the dominant
allele
– The other allele, which has no noticeable effect on the appearance, is
the recessive allele
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4. Gametes carry only one allele for each inherited character.
– The two members of an allele pair segregate (separate) from each
other during the production of gametes.
– This statement is the law of segregation.
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P Generation Genetic makeup (alleles)
Purple flowers
Alleles carried PP
White flowers
pp
by parents
Gametes
All P
All p
Figure 9.6-1
P Generation Genetic makeup (alleles)
Purple flowers
Alleles carried PP
White flowers
pp
by parents
Gametes
All P
All p
F1 Generation
(hybrids)
Purple flowers
All Pp
Alleles
segregate
Gametes
1
P
2
1 p
2
Figure 9.6-2
P Generation Genetic makeup (alleles)
Purple flowers
Alleles carried PP
White flowers
pp
by parents
Gametes
All p
All P
F1 Generation
(hybrids)
Purple flowers
All Pp
Alleles
segregate
Gametes
1
P
2
F2 Generation
(hybrids)
Sperm from
F1 plant
p
P
P
Eggs from
F1 plant
1 p
2
PP
Pp
Pp
pp
p
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
Figure 9.6-3
• Geneticists distinguish between an organism’s physical traits and
its genetic makeup.
– An organism’s physical traits are its phenotype.
– An organism’s genetic makeup is its genotype.
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Mendel’s Law of Independent Assortment
• A dihybrid cross is the crossing of parental varieties differing in
two characters.
• What would result from a dihybrid cross? Two hypotheses are
possible:
1. Dependent assortment
2. Independent assortment
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(a) Hypothesis: Dependent assortment
(b) Hypothesis: Independent assortment
P Generation
RRYY
RRYY
rryy
Gametes RY
ry
F1 Generation
rryy
Gametes RY
RrYy
ry
RrYy
Sperm
F2 Generation
1
1
rY
RY
4
4
Sperm
1
2 RY
1
ry
2
1
2 RY
Eggs
1
ry
2
Predicted results
(not actually seen)
1
Ry 1 ry
4
4
1
RY
4
RRYY RrYY RRYy RrYy
1
rY
4
RrYY rrYY
RrYy rrYy
9
16
1 Ry
4
RRYy RrYy RRyy Rryy
3
16
1 ry
4
3
16
1
16
Eggs
RrYy rrYy Rryy rryy
Actual results
(support hypothesis)
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
Figure 9.8
Blind dog
Phenotypes
Black coat,
Black coat,
blind (PRA)
normal vision
B_N_
Genotypes
B_nn
(a) Possible phenotypes of Labrador retrievers
Blind dog
Chocolate coat,
normal vision
bbN_
Chocolate coat,
blind (PRA)
bbnn
Mating of double heterozygotes
(black coat, normal vision)
BbNn
BbNn
Phenotypic
ratio of
offspring
9 black coat,
normal vision
3 black coat,
blind (PRA)
3 chocolate coat,
normal vision
1 chocolate coat,
blind (PRA)
(b) A Labrador dihybrid cross
Figure 9.9
Using a Testcross to Determine an Unknown
Genotype
• A testcross is a mating between
– An individual of dominant phenotype (but unknown genotype)
– A homozygous recessive individual
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Phenotype
Genotype
Recessive
pp
Dominant
P?
or
Phenotype
All dominant
Conclusion
Unknown parent
is PP
1 dominant : 1 recessive
Unknown parent
is Pp
Figure 9.UN2
Testcross
Genotypes
B_
bb
Two possible genotypes for the black dog:
BB
Gametes
B
Offspring
b Bb
All black
or
Bb
B
b
b Bb bb
1 black : 1 chocolate
Figure 9.10
Family Pedigrees
• Mendel’s principles apply to the inheritance of many human
traits.
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• Dominant traits are not necessarily
– Normal or
– More common
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DOMINANT TRAITS
Widow’s peak
Free earlobe
No freckles
Straight hairline
Attached earlobe
RECESSIVE TRAITS
Freckles
Figure 9.12
• A family pedigree
– Shows the history of a trait in a family
– Allows geneticists to analyze human traits
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First generation
(grandparents)
Second generation
(parents, aunts, and
uncles)
Third generation
(brother and
sister)
Female Male
Attached
Free
Ff
FF
or
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Ff
ff
Figure 9.13
Recessive Disorders
• Most human genetic disorders are recessive.
• Individuals who have the recessive allele but appear normal are
carriers of the disorder.
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Table 9.1
• Cystic fibrosis
– Is the most common lethal genetic disease in the United States
– Is caused by a recessive allele carried by about one in 25 people of
European ancestry
• Prolonged geographic isolation of certain populations can lead to
inbreeding, the mating of close relatives.
– Inbreeding increases the chance of offspring that are homozygous for a
harmful recessive trait.
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Genetic Testing
• Today many tests can detect the presence of disease-causing
alleles.
• Most genetic testing is performed during pregnancy.
– Amniocentesis collects cells from amniotic fluid.
– Chorionic villus sampling removes cells from placental tissue.
• Genetic counseling helps patients understand the results and
implications of genetic testing.
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VARIATIONS ON MENDEL’S LAWS
• Some patterns of genetic inheritance are not explained by
Mendel’s laws.
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Incomplete Dominance in Plants and People
• In incomplete dominance, F1 hybrids have an appearance in
between the phenotypes of the two parents.
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P Generation
White
rr
Red
RR
Gametes
R
r
Figure 9.18-1
P Generation
White
rr
Red
RR
Gametes
R
r
F1 Generation
Pink
Rr
Gametes
1
1
R
2 r
2
Sperm
1
1
R
2 r
2
F2 Generation
1
2 R
Eggs
1
r
2
RR
Rr
Rr
rr
Figure 9.18-3
Polygenic Inheritance
• Polygenic inheritance is the additive effects of two or more
genes on a single phenotype.
Polygenic
inheritance
Single trait
(e.g., skin
color)
Multiple genes
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P Generation
aabbcc
(very light)
AABBCC
(very dark)
AaBbCc
AaBbCc
F1 Generation
F2 Generation
1
8
1
8
1
8
Sperm
1
1
8
8
1
8
1
8
1
8
Fraction of population
1
8
1
8
1
8
1
Eggs 8
1
8
1
8
1
8
1
8
20
64
15
64
6
64
1
64
Skin pigmentation
1
64
6
64
15
64
20
64
15
64
6
64
1
64
Figure 9.22
The Role of Environment
• Many human characters result from a combination of heredity and
environment.
• Only genetic influences are inherited.
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THE CHROMOSOMAL BASIS OF
INHERITANCE
• The chromosome theory of inheritance states that
– Genes are located at specific positions on chromosomes
– The behavior of chromosomes during meiosis and fertilization accounts
for inheritance patterns
• It is chromosomes that undergo segregation and independent
assortment during meiosis and thus account for Mendel’s laws.
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A
B
a
b
A B
Parental gametes
a b
A
Pair of
homologous
chromosomes
Crossing over
b
a
B
Recombinant gametes
Figure 9.26
SEX CHROMOSOMES AND SEX-LINKED
GENES
• Sex chromosomes influence the inheritance of certain traits.
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Sex Determination in Humans
• Nearly all mammals have a pair of sex chromosomes designated
X and Y.
– Males have an X and Y.
– Females have XX.
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Male
Female
44

XY
Somatic
cells
22

Y
22

X
Sperm
44

XX
Offspring
Female
44

XX
22

X
Egg
44

XY
Male
Figure 9.29
X
Colorized SEM
Y
Figure 9.29
Sex-Linked Genes
• Any gene located on a sex chromosome is called a sex-linked
gene.
– Most sex-linked genes are found on the X chromosome.
– Red-green color blindness is a common human sex-linked disorder.
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Figure 9.30
XNXN
XnY
XNXn
XNY
Sperm
Eggs XN
XN
X nY
Sperm
Sperm
Y
Y
Xn
Y
XNXn
XNY
Eggs XN
N
XNXN X Y
Eggs XN
N
XNXn X Y
N
XNXn X Y
Xn
n
XNXn X Y
Xn
n
XnXn X Y
XN
(a)
Normal female

colorblind male
Key
XNXn
Unaffected individual
Xn
(b)
Carrier female

normal male
Carrier
(c)
Carrier female

colorblind male
Colorblind individual
Figure 9.31
• Hemophilia
– Is a sex-linked recessive blood-clotting trait that may result in excessive
bleeding and death after relatively minor cuts and bruises
– Has plagued royal families of Europe
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Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.32
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A true-breeding plant that produces yellow seeds is crossed
with a true-breeding plant that produces green seeds. The F1
plants have yellow seeds. What is the expected phenotypic
ratio of seed color of the offspring of an F1 × F1 cross?
A) 1:2:1
B) 2:1
C) 3:1
D) 9:3:3:1
E) 1:1
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A true-breeding plant that produces yellow seeds is crossed
with a true-breeding plant that produces green seeds. The F1
plants have yellow seeds. What is the expected phenotypic
ratio of seed color of the offspring of an F1 × F1 cross?
A) 1:2:1
B) 2:1
C) 3:1
D) 9:3:3:1
E) 1:1
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Alleles are described as ______.
A) homologous chromosomes
B) environmental factors that affect gene expression
C) alternate versions of a gene
D) Punnett squares
E) alternate phenotypes
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Alleles are described as ______.
A) homologous chromosomes
B) environmental factors that affect gene expression
C) alternate versions of a gene
D) Punnett squares
E) alternate phenotypes
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A true-breeding plant that produces yellow seeds is
crossed with a true-breeding plant that produces green
seeds. The seeds of all of the offspring are yellow.
Why?
A) The yellow allele is recessive to the green allele.
B) All of the offspring are homozygous yellow.
C) The yellow allele is dominant to the green allele.
D) The alleles are codominant.
E) Yellow is an easier color to produce.
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A true-breeding plant that produces yellow seeds is
crossed with a true-breeding plant that produces green
seeds. The seeds of all of the offspring are yellow.
Why?
A) The yellow allele is recessive to the green allele.
B) All of the offspring are homozygous yellow.
C) The yellow allele is dominant to the green allele.
D) The alleles are codominant.
E) Yellow is an easier color to produce.
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In humans, the presence or absence of dimples is a
trait controlled by a single gene. What is the genotype
of an individual who is heterozygous for dimples?
A) dimples
B) DD
C) Dd
D) dd
E) DI
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In humans, the presence or absence of dimples is a
trait controlled by a single gene. What is the genotype
of an individual who is heterozygous for dimples?
A) dimples
B) DD
C) Dd
D) dd
E) DI
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Which of these crosses will only produce
heterozygous offspring?
A) AA × aa
B) AA × Aa
C) Aa × Aa
D) aa × aa
E) Aa × aa
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Which of these crosses will only produce
heterozygous offspring?
A) AA × aa
B) AA × Aa
C) Aa × Aa
D) aa × aa
E) Aa × aa
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To determine the phenotype of an individual who
expresses a dominant trait, you would cross that
individual with an individual who ______.
A) expresses the dominant trait
B) is homozygous recessive for that trait
C) has the genotype Aa
D) is homozygous dominant for that trait
E) is heterozygous for that trait
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To determine the phenotype of an individual who
expresses a dominant trait, you would cross that
individual with an individual who ______.
A) expresses the dominant trait
B) is homozygous recessive for that trait
C) has the genotype Aa
D) is homozygous dominant for that trait
E) is heterozygous for that trait
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A couple has two female children. What is the
probability that their next child will be male?
A) 25%
B) 50%
C) 75%
D) 33%
E) 67%
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A couple has two female children. What is the
probability that their next child will be male?
A) 25%
B) 50%
C) 75%
D) 33%
E) 67%
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An individual with (naturally) curly hair and an
individual with (naturally) straight hair mate; all of
their offspring have (naturally) wavy hair. What is the
relationship between the alleles for hair texture?
A) pleiotropy
B) incomplete dominance
C) straight hair and curly hair are sex-linked, but wavy
hair is not
D) wavy hair is dominant to both straight and curly
hair
E) codominance
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An individual with (naturally) curly hair and an
individual with (naturally) straight hair mate; all of
their offspring have (naturally) wavy hair. What is the
relationship between the alleles for hair texture?
A) pleiotropy
B) incomplete dominance
C) straight hair and curly hair are sex-linked, but wavy
hair is not
D) wavy hair is dominant to both straight and curly
hair
E) codominance
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An individual with (naturally) curly hair and an
individual with (naturally) straight hair mate; all of
their offspring have (naturally) wavy hair. If an
individual with wavy hair mates with an individual
with straight hair, what is the probability that their
child will have curly hair?
A) 0%
B) 25%
C) 50%
D) 75%
E) 100%
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An individual with (naturally) curly hair and an
individual with (naturally) straight hair mate; all of
their offspring have (naturally) wavy hair. If an
individual with wavy hair mates with an individual
with straight hair, what is the probability that their
child will have curly hair?
A) 0%
B) 25%
C) 50%
D) 75%
E) 100%
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Chromosomes that are not sex chromosomes are called
______.
A) centrosomes
B) allosomes
C) nonsexosomes
D) autosomes
E) dominant
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Chromosomes that are not sex chromosomes are called
______.
A) centrosomes
B) allosomes
C) nonsexosomes
D) autosomes
E) dominant
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The parents of a child with unusual disease symptoms take the child to a
doctor for help. The doctor suspects that the condition might have a
genetic basis. She recommends that the child be taken to a specialty
clinic where physicians and staff members are trained to diagnose
genetic diseases and counsel parents. Ultimately, the child is diagnosed
with a rare recessively inherited disease. The parents are tested for the
gene, and both are found to be heterozygous. The parents want to have
another child but are afraid this child will also be affected.
4) What would genetic counselors say is the probability that the second
child will have the disease?
A) 1/2
B) 1/4
C) 1/8
D) 1/16
E) 1/32
© 2010 Pearson Education, Inc.
The parents of a child with unusual disease symptoms take the child to a
doctor for help. The doctor suspects that the condition might have a
genetic basis. She recommends that the child be taken to a specialty
clinic where physicians and staff members are trained to diagnose
genetic diseases and counsel parents. Ultimately, the child is diagnosed
with a rare recessively inherited disease. The parents are tested for the
gene, and both are found to be heterozygous. The parents want to have
another child but are afraid this child will also be affected.
4) What would genetic counselors say is the probability that the second
child will have the disease?
A) 1/2
B) 1/4
C) 1/8
D) 1/16
E) 1/32
© 2010 Pearson Education, Inc.