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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.
Biology And Society:
A Matter of Breeding
• Genetics is the scientific study of heredity.
– Genetics explains why the offspring of purebred dogs are like their
parents.
– Inbreeding of dogs makes some genetic disorders common.
• A dog’s behavior is determined by its
– Genes
– Environment
© 2010 Pearson Education, Inc.
Figure 9.00
HERITABLE VARIATION AND PATTERNS
OF INHERITANCE
• Heredity is the transmission of traits from one generation to
the next.
• Genetics is the scientific study of heredity.
• 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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Figure 9.1
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
• A character is a heritable feature that varies among individuals; like
color.
• A trait is a variant of a character; like purple or white color.
• Each of the characters Mendel studied occurred in two distinct forms.
• Mendel
– Created true-breeding varieties of plants (purebred)
– 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.
© 2010 Pearson Education, Inc.
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.
Figure 9.3-2
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, purple or white, etc.
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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
Monohybrid Crosses
• A monohybrid cross is a cross between parent plants that differ in
only one character.
© 2010 Pearson Education, Inc.
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.
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. When a sperm and egg
unite at fertilization, each contributes its alleles, restoring the paired
condition in the offspring. Figure 9.6
© 2010 Pearson Education, Inc.
• Do Mendel’s hypotheses account for the 3:1 ratio he observed in
the F2 generation?
• A Punnett square highlights the four possible combinations of
gametes and offspring that result from each cross.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
Genetic Alleles and Homologous Chromosomes
• Homologous chromosomes have
– Genes at specific loci
– Alleles of a gene at the same locus, the same positions along their lengths.
© 2010 Pearson Education, Inc.
Gene loci
Homologous
chromosomes
Dominant
allele
P
a
B
P
a
b
Recessive
allele
Genotype: PP
Homozygous
for the
dominant allele
aa
Homozygous
for the
recessive allele
Bb
Heterozygous
Figure 9.7
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 characters were inherited together (3:1), 2
gametes.
2. Independent assortment: the inheritance of one character has no effect on
the inhert. of another. 4 gametes in equal quantities.
* Dihybrid cross = 2 monohybrid crosses occurring simultaneously (9:3:3:1)
© 2010 Pearson Education, Inc.
(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
Figure 9.8
• Mendel’s dihybrid cross supported the hypothesis that each pair
of alleles segregates independently of the other pairs during
gamete formation.
• Thus, the inheritance of one character has no effect on the
inheritance of another.
• This is the law of independent assortment.
• Independent assortment is also seen in two hereditary characters
in Labrador retrievers.
© 2010 Pearson Education, Inc.
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
Blind dog
Blind dog
Phenotypes
Genotypes
Black coat,
normal vision
B_N_
Black coat,
blind (PRA)
B_nn
Chocolate coat,
normal vision
bbN_
Chocolate coat,
blind (PRA)
bbnn
(a) Possible phenotypes of Labrador retrievers
Figure 9.9a
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, 1 chocolate coat,
blind (PRA)
normal vision
(b) A Labrador dihybrid cross
Figure 9.9b
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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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
The Rules of Probability
• Mendel’s strong background in mathematics helped him
understand patterns of inheritance.
• The rule of multiplication states that the probability of a
compound event is the product of the separate probabilities of the
independent events.
• What is the chance that both coins will land heads-up? The
probability of such a dual event is the product of the separate
probabilities of the independent events – for the coins; ½ * ½ =
¼. Figure 9.11
© 2010 Pearson Education, Inc.
F1 Genotypes
Bb female
Bb male
Formation of sperm
Formation of eggs
F2 Genotypes
Male gametes
Female gametes
1
2
1
2
1
2
B
B
b
1
2
B
B
b
B
1
1 4 1
( 2 2 )
B
b
1
4
b
1
4
b
b
1
4
Figure 9.11
52 deck of cards, what is the probability of being
dealt an ace?
• 4 aces/ 52 cards = 1/13
• Ace and King? 4/52 + 4/52 = 2/13
• Ace then another Ace? 4/52 * 3/51 = 1/221
© 2010 Pearson Education, Inc.
Family Pedigrees
• Mendel’s principles apply to the inheritance of many human
traits.
• Then researchers assembles this information into a family tree.
• He used concepts of dominant/recessive and law of segregation.
© 2010 Pearson Education, Inc.
DOMINANT TRAITS
Widow’s peak
Free earlobe
No freckles
Straight hairline
Attached earlobe
RECESSIVE TRAITS
Freckles
Figure 9.12
• Dominant traits are not necessarily
– Normal or
– More common
• Wild-type traits are
– Those seen most often in nature
– Not necessarily specified by dominant alleles
• A family pedigree
– Shows the history of a trait in a family
– Allows geneticists to analyze human traits
© 2010 Pearson Education, Inc.
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
Human Disorders Controlled by a Single Gene
• Many human traits
– Show simple inheritance patterns
– Are controlled by single genes on autosomes
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Table 9.1
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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Parents
Hearing
Dd
Hearing
Dd
Offspring
D
D
Sperm
d
DD
Hearing
Dd
Hearing
(carrier)
Dd
Hearing
(carrier)
dd
Deaf
Eggs
d
Figure 9.14
• 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.
© 2010 Pearson Education, Inc.
Dominant Disorders
• Some human genetic disorders are dominant.
– Huntington’s disease, which leads to degeneration of the nervous
system, does not begin until middle age.
– Achondroplasia is a form of dwarfism.
–
The homozygous dominant genotype causes death of the embryo.
–
Thus, only heterozygotes have this disorder.
© 2010 Pearson Education, Inc.
Parents
Dwarf
Normal
(achondroplasia)
(no achondroplasia)
Dd
dd
d
D
Sperm
d
Dd
Dwarf
Dd
Dwarf
dd
Normal
dd
Normal
Eggs
d
Molly Jo
Matt
Amy
Zachary
Jake
Jeremy
Figure 9.16
The Process of Science:
What Is the Genetic Basis of Hairless Dogs?
• Observation: Dogs come in a wide variety of physical types.
• Question: What is the genetic basis for the hairless phenotype?
• Hypothesis: A comparison of genes of coated and hairless dogs
would identify the gene or genes responsible.
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• Prediction: A mutation in a single gene accounts for the hairless
appearance.
• Experiment: Compared DNA sequences of 140 hairless dogs
from 3 breeds with 87 coated dogs from 22 breeds.
• Results: Every hairless dog, but no coated dogs, had a single
change in a single gene.
© 2010 Pearson Education, Inc.
Figure 9.17
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.
© 2010 Pearson Education, Inc.
Incomplete Dominance in Plants and People
• In incomplete dominance, F1 hybrids have an appearance in
between the phenotypes of the two parents.
© 2010 Pearson Education, Inc.
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
Figure 9.18-2
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
• Hypercholesterolemia
– Is characterized by dangerously high levels of cholesterol in the blood.
– Is a human trait that is incompletely dominant.
– Heterozygotes have blood cholesterol levels about twice normal.
– Homozygotes have blood cholesterol levels about five times normal.
© 2010 Pearson Education, Inc.
GENOTYPE
PHENOTYPE
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
Mild disease
Severe disease
LDL
LDL
receptor
Cell
Normal
Figure 9.19
ABO Blood Groups: An Example of Multiple
Alleles and Codominance
• The ABO blood groups in humans are an example of multiple
alleles.
• The immune system produces blood proteins called antibodies
that can bind specifically to blood cell carbohydrates.
• Blood cells may clump together if blood cells of a different type
enter the body.
• The clumping reaction is the basis of a blood-typing lab test.
© 2010 Pearson Education, Inc.
Blood
Antibodies Reactions When Blood from Groups Below Is
Group
Genotypes Red Blood Cells Present in Mixed with Antibodies from Groups at Left
(Phenotype)
Blood
A
B
AB
O
Carbohydrate A
IAIA
Anti-B
A
or
IAi
Carbohydrate B
IBIB
B
Anti-A
or
IBi
AB
IAIB
—
O
ii
Anti-A
Anti-B
Figure 9.20
Blood
Group
Genotypes
(Phenotype)
A
IAIA
or
IAi
Red Blood Cells
Carbohydrate A
Carbohydrate B
B
IBIB
or
IBi
AB
IAIB
O
ii
Figure 9.20a
Antibodies
Present in
Blood
Reactions When Blood from Groups Below Is
Mixed with Antibodies from Groups at Left
O
B
AB
A
Anti-B
Anti-A
—
Anti-A
Anti-B
Figure 9.20b
Figure 9.20
Individual homozygous,
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Colorized SEM
Abnormal hemoglobin crystallizes into long flexible chains,
causing red blood cells to become sickle-shaped.
Sickled cells can lead to a cascade of symptoms, such as
weakness, pain, organ damage, and paralysis.
Figure 9.21
Figure 9.21
• The human blood type alleles IA and IB exhibit codominance:
Both alleles are expressed in the phenotype.
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Pleiotropy and Sickle-Cell Disease
• Pleiotropy is the impact of a single gene on more than one
character.
• Sickle-cell disease
– Exhibits pleiotropy
– Results in abnormal hemoglobin production
– Causes disk-shaped red blood cells to deform into a sickle shape with
jagged edges
– Characterized by a diverse set of symptoms
– Cells link together and crystallize, so O2 is low.
– Clogging of vessels
© 2010 Pearson Education, Inc.
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
Polygenic Inheritance
• Polygenic inheritance is the additive effects of two or more genes on a
single phenotype.
• The opposite of pleiotropy, where one gene affects several characters.
• Skin color is controlled by 3 genes that are inherited separately, the
peas.
• Dark skin for each gene (A, B & C) contributes one “unit” of darkness
to the phenotype and is incompletely dominant to the other alleles (a, b
& c).
• A person who is AABBCC would be very dark, while aabbcc would be
very light.
• AaBbCc = intermediate shade b/c of the additive effect, thus AaBbCc
have the same skin color as AABbcc, three dark skin alleles.
© 2010 Pearson Education, Inc.
P Generation
aabbcc
(very light)
AABBCC
(very dark)
AaBbCc
AaBbCc
F1 Generation
F2 Generation
1
8
1
8
1
64
6
64
1
8
Sperm
1
1
8
8
1
8
1
8
1
8
1
8
1
8
1
8
1
Eggs 8
1
8
1
8
1
8
1
8
15
64
20
64
15
64
6
64
1
64
Figure 9.22a
Fraction of population
20
64
15
64
6
64
1
64
Skin pigmentation
Figure 9.22b
The Role of Environment
• Many human characters result from a combination of heredity and
environment.
• Only genetic influences are inherited.
• As a result of environ. influences, even identical twins can look
different. (phenotype)
• Any effects of environment are not passed on to the next gen.
© 2010 Pearson Education, Inc.
P Generation
Round-yellow seeds
(RRYY)
r y
Y
Y
Wrinkled-green seeds
(rryy)
R R
y
r
MEIOSIS
FERTILIZATION
Gametes
y
R Y
F1 Generation
R
r
r
All round-yellow seeds
(RrYy)
y
Figure 9.24-1
P Generation
Round-yellow seeds
(RRYY)
r y
Y
Y
Wrinkled-green seeds
(rryy)
R R
y
r
MEIOSIS
FERTILIZATION
Gametes
y
R Y
F1 Generation
R
r
Law of Segregation: Follow the long
chromosomes (carrying R and r) taking
either the left or right branch.
R
Y
r
All round-yellow seeds
(RrYy)
y
Law of Independent Assortment:
Follow both the long and the short
chromosomes.
Y
MEIOSIS
r
y
Metaphase I
(alternative
arrangements)
r
Y
R
y
They are arranged in either of
two equally likely ways at
metaphase I.
Figure 9.24-2
P Generation
Round-yellow seeds
(RRYY)
r y
Y
R R
Y
Wrinkled-green seeds
(rryy)
y
r
MEIOSIS
FERTILIZATION
Gametes
y
R Y
F1 Generation
R
r
Law of Segregation: Follow the long
chromosomes (carrying R and r) taking
either the left or right branch.
R
The R and r alleles segregate in
anaphase I of meiosis.
Only one long
chromosome ends
up in each gamete.
Gametes
Y
y
Y
y
1
RY
4
Law of Independent Assortment:
Follow both the long and the short
chromosomes.
Metaphase I
(alternative
arrangements)
r
r
R
y
MEIOSIS
y
Y
R
All round-yellow seeds
(RrYy)
Y
R
Y
r
Metaphase
II
r
1
ry
4
Y
R
They are arranged in either of
two equally likely ways at
metaphase I.
y
r
R
Y
y
Y
y
r
r
Y
r
r
1
rY
4
They sort independently,
giving four gamete types.
y
y
R
R
1
Ry
4
Figure 9.24-3
P Generation
Round-yellow seeds
(RRYY)
r y
Y
R R
Y
Wrinkled-green seeds
(rryy)
y
r
MEIOSIS
FERTILIZATION
Gametes
y
R Y
F1 Generation
R
r
Law of Segregation: Follow the long
chromosomes (carrying R and r) taking
either the left or right branch.
R
The R and r alleles segregate in
anaphase I of meiosis.
Only one long
chromosome ends
up in each gamete.
Gametes
Y
F2 Generation
Law of Independent Assortment:
Follow both the long and the short
chromosomes.
Metaphase I
(alternative
arrangements)
r
y
r
Y
y
y
R
Metaphase
II
r
Y
1
ry
4
y
Y
y
Y
r
1
rY
4
FERTILIZATION AMONG THE F1 PLANTS
9
:3
:3
They are arranged in either of
two equally likely ways at
metaphase I.
R
r
r
R
r
Y
y
r
1
RY
4
Fertilization recombines the r
and R alleles at random.
y
MEIOSIS
Y
R
All round-yellow seeds
(RrYy)
Y
R
Y
r
They sort independently,
giving four gamete types.
y
y
R
R
1
Ry
4
Fertilization results in the
9:3:3:1 phenotypic ratio in
the F2 generation.
:1
Figure 9.24-4
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.
© 2010 Pearson Education, Inc.
Dihybrid testcross
Gray body,
long wings
(wild-type)
Black body,
short wings
(mutant)
GgLl
ggll
Female
Male
Figure 9.25-1
Dihybrid testcross
Gray body,
long wings
(wild-type)
Black body,
short wings
(mutant)
GgLl
ggll
Female
Male
Results
Offspring
Gray-long
GgLl
965
Black-short
ggll
944
Parental phenotypes 83%
Gray-short
Ggll
206
Black-long
ggLl
185
Recombinant phenotypes 17%
Figure 9.25-2
Linked Genes
• Linked genes
– Are located close together on a chromosome
– May be inherited together
• Using the fruit fly Drosophila melanogaster, Thomas Hunt
Morgan determined that some genes were linked based on the
inheritance patterns of their traits.
© 2010 Pearson Education, Inc.
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
Genetic Recombination: Crossing Over
• Crossing over can
– Separate linked alleles
– Produce gametes with recombinant chromosomes
– Produce offspring with recombinant phenotypes
© 2010 Pearson Education, Inc.
GgLl
(female)
GL
gl
gl
gl
ggll
(male)
Crossing over
GL
Gl
gl
gL
gl
Sperm
Parental gametes
Recombinant gametes
Eggs
FERTILIZATION
Offspring
GL
gl
Gl
gL
gl
gl
gl
gl
Parental
Recombinant
Figure 9.27
g l
G L
ggll
(male)
GgLl
(female)
g l
g l
Crossing over
G L
gl
Gl
g L
gl
Sperm
Parental gametes
Recombinant gametes
Eggs
Figure 9.27a
GL
Gl
gl
gL
gl
Sperm
Parental gametes
Recombinant gametes
Eggs
FERTILIZATION
Offspring
G L
gl
Gl
g L
gl
gl
gl
gl
Parental
Recombinant
Figure 9.27b
• The percentage of recombinant offspring among the total is called
the recombination frequency.
© 2010 Pearson Education, Inc.
Chromosome
g
c
l
17%
9%
9.5%
Recombination
frequencies
Figure 9.28
Linkage Maps
• Early studies of crossing over were performed using the fruit fly
Drosophila melanogaster.
• Alfred H. Sturtevant, a student of Morgan, developed a method
for mapping gene loci, which resulted in the creation of linkage
maps.
– A diagram of relative gene locations on a chromosome is a linkage map.
© 2010 Pearson Education, Inc.
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 CHROMOSOMES AND SEX-LINKED
GENES
• Sex chromosomes influence the inheritance of certain traits.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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.
– Can you see a green number 7 against the red background? If not, you
probably have some form of red-green colorblindness.
© 2010 Pearson Education, Inc.
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
© 2010 Pearson Education, Inc.
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.32
Evolution Connection:
Barking Up the Evolutionary Tree
• About 15,000 years ago in East Asia, humans began to cohabit
with ancestral canines that were predecessors of modern wolves
and dogs.
• As people settled into geographically distinct populations,
different canines became separated and inbred.
• In 2005 researchers sequenced the complete genome of a dog.
• An evolutionary tree of dog breeds was created.
© 2010 Pearson Education, Inc.
Wolf
Chinese shar-pei
Ancestral
canine
Akita
Basenji
Siberian husky
Alaskan
malamute
Afghan hound
Saluki
Rottweiler
Sheepdog
Retriever
Figure 9.33
Fertilization
Alleles
Meiosis
Diploid cell
(contains paired
alleles, alternate
forms of a gene)
Gamete
from other
parent
Diploid zygote
(contains
paired alleles)
Haploid gametes
(allele pairs separate)
Figure 9.UN1
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
Dominant phenotype
(RR)
Recessive phenotype
(rr)
Intermediate phenotype
(incomplete dominance)
(Rr)
Figure 9.UN3
Single
gene
Pleiotropy
Multiple traits
(e.g., sickle-cell
disease)
Figure 9.UN4
Polygenic
inheritance
Single trait
(e.g., skin color)
Multiple genes
Figure 9.UN5
Male
44

XY
Female
Somatic
cells
44

XX
Figure 9.UN6
Figure 9.UN7