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BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 9
Patterns of Inheritance
Modules 9.1 – 9.10
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Purebreds and Mutts — A Difference of Heredity
• Genetics is the science of heredity
• These black Labrador puppies are purebred—
their parents and grandparents were black Labs
with very similar genetic makeups
– Purebreds
often suffer
from serious
genetic defects
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• The parents of these puppies were a mixture of
different breeds
– Their behavior
and appearance
is more varied
as a result of
their diverse
genetic
inheritance
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MENDEL’S PRINCIPLES
The science of genetics has ancient roots
• The science of heredity dates back to ancient
attempts at selective breeding
• Until the 20th century, however, many
biologists erroneously (wrongly!) believed that
– characteristics acquired during lifetime could be
passed on
– characteristics of both parents blended
irreversibly in their offspring
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Experimental genetics began in an abbey garden
• Modern genetics began with Gregor Mendel’s
quantitative experiments with pea plants
Stamen
Carpel
Figure 9.2A, B
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• Mendel crossed
pea plants that
differed in certain
characteristics and
traced the traits
from generation to
generation
• This illustration
shows his
technique for
cross-fertilization
White
1
Removed
stamens
from purple
flower
Stamens
Carpel
PARENTS
(P)
2 Transferred
Purple
pollen from
stamens of white
flower to carpel
of purple flower
3 Pollinated carpel
matured into pod
4
OFFSPRING
(F1)
Figure 9.2C
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Planted
seeds
from pod
• Mendel studied
seven pea
characteristics
FLOWER
COLOR
Purple
White
Axial
Terminal
SEED
COLOR
Yellow
Green
SEED
SHAPE
Round
Wrinkled
POD
SHAPE
Inflated
Constricted
POD
COLOR
Green
Yellow
STEM
LENGTH
Tall
Dwarf
FLOWER
POSITION
• He hypothesized
that there are
alternative forms
of genes – alleles
(although he did not
use that term), the
units that
determine
heredity
Figure 9.2D
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Mendel’s principle of segregation describes the
inheritance of a single characteristic
• From his
experimental data,
Mendel deduced
that an organism
has two genes for
each inherited
characteristic
P GENERATION
(true-breeding
parents)
Purple flowers
White flowers
All plants have
purple flowers
F1
generation
Fertilization
among F1
plants
(F1 x F1)
– One characteristic
comes from each
parent
F2
generation
Figure 9.3A
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3/
of plants
have purple flowers
4
1/
4 of plants
have white flowers
GENETIC MAKEUP (ALLELES)
• A sperm or egg
carries only one
gene of each pair
P PLANTS
Gametes
– The pairs of genes
separate when
gametes form
PP
pp
All P
All p
F1 PLANTS
(hybrids)
Gametes
– This process
describes Mendel’s
law of segregation
All Pp
1/
2
1/
P
P
2
p
P
Eggs
Sperm
PP
F2 PLANTS
– Alleles can be
dominant or
recessive
Phenotypic ratio
3 purple : 1 white
p
p
Pp
Pp
pp
Genotypic ratio
1 PP : 2 Pp : 1 pp
Figure 9.3B
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Principle of Segregation
• Homologous pairs of
genes segregate
(separate) during
gamete formation
(meiosis).
• The joining of gametes
at fertilization pair the
genes once again.
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Homologous chromosomes bear the two alleles for
each characteristic
• Alternative forms of a gene (alleles) reside at
the same locus on homologous chromosomes
GENE LOCI
P
P
a
a
B
DOMINANT
allele
b
RECESSIVE
allele
GENOTYPE:
PP
aa
HOMOZYGOUS
for the
dominant allele
HOMOZYGOUS
for the
recessive allele
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Bb
HETEROZYGOUS
Figure 9.4
Genetic Vocabulary
Gene – a segment of DNA that contains the
instructions that code for a particular trait
Locus – specific location of a gene on a
chromosome
Allele – alternate versions of a gene at a single
locus
Homozygous – when the alleles of a gene are the
same on the homologous chromosomes
Heterozygous – when the alleles of a gene are
different on the homologous chromosomes
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Genetics Vocabulary
Dominant – the allele that is expressed when the
alleles are heterozygous.
Represented by an upper case letter
Recessive – the allele that is not expressed when
the alleles are heterozygous.
Represented by a lower case letter.
To be expressed the cell must have 2 copies of
the recessive allele
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Genetics Vocabulary
Phenotype – the physical appearance of a trait in
an organism
Genotype – the genetic make up of an organism
with respect to a trait.
The genotype of a trait can be homozygous
dominant, heterozygous or homozygous
recessive
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Mendel’s principles reflect the rules of probability
• Inheritance follows
the rules of probability
F1 GENOTYPES
Bb female
Bb male
Formation of eggs
Formation of sperm
1/
B
1/
2
B
2
B
B
1/
b
1/
1/
2
b
B
b
1/
4
b
b
4
B
1/
2
4
b
F2 GENOTYPES
1/
4
Figure 9.7
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Predicting the Outcome of a Monohybrid Cross
Predict the results of the following cross (using R
to denote tongue-rolling ability):
P generation: RR x RR
1. What genotype(s) will be found in the F1
generation?
2. What phenotype(s) will be found in the F1
generation?
3. Explain why you made these predictions.
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Predicting the Outcomes of a Monohybrid Cross
Predict the results of the following cross:
P generation: RR x rr
1. What genotype(s) will be found in the F1
generation?
2. What phenotype(s) will be found in the F1
generation?
3. Explain why you made these predictions.
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Predicting the Outcome of a Monohybrid Cross
Predict the results of the following cross:
P generation = Rr x Rr
1. Draw the Punnett square.
2. What are the possible genotypes in the F1
generation?
3. What is the genotypic ratio of this cross?
4. What are the possible phenotypes in the F1
generation?
5. What is the phenotypic ratio for this cross?
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Geneticists use the testcross to determine unknown
genotypes
• The offspring of a testcross often reveal the
genotype of an individual when it is unknown
TESTCROSS:
GENOTYPES
B_
bb
Two possibilities for the black dog:
BB
b
OFFSPRING
Bb
B
GAMETES
Figure 9.6
or
Bb
All black
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B
b
Bb
b
bb
1 black : 1 chocolate
Test cross
• A testcross is the mating between an individual
of unknown genotype with a homozygous
recessive genotype.
• Usually performed when the phenotype of the
unknown individual is dominant.
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Understanding Test Cross
1. Brown coat color (B) in rabbits is dominant
and white coat color is recessive. Suppose you
have a group of rabbits – some brown and
some white.
a. For which phenotype(s) do you know the
genotype(s)?
b. For which phenotype(s) are you unsure of the
genotype(s)?
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Understanding Test Cross
•
Using B and b to symbolize the brown and
white alleles
a. What are the possible genotypes of a white
rabbit in your group?
b. What are the possible genotypes of a brown
rabbit?
•
Suppose you wanted to find out the genotype
of a brown rabbit. What color rabbit would
you mate it with?
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Understanding Test Cross
•
A brown buck (male) is mated with a white
doe (female). In their litter of 11 young, 6 are
white and 5 are brown.
a. Using a Punnett square to check your answer,
what is the genotype of the buck?
•
Use a Punnett square to determine the ratio of
brown and white offspring that would have
been produced by the above mating if the
brown buck had been homozygous.
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HYPOTHESIS:
DEPENDENT ASSORTMENT
RRYY
P
GENERATION
rryy
Gametes
RRYY
ry
RY
rryy
Gametes
ry
RY
RrYy
F1
GENERATION
Eggs
1/
HYPOTHESIS:
INDEPENDENT ASSORTMENT
2
1/
2
RY
1/
2
RrYy
RY
1/
ry
Sperm
2
1/
ry
1/
F2
GENERATION
1/
Eggs
1/
4
4
4
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RY
1/
4
RrYy
rY
1/
RrYY
rrYY
rrYy
Figure 9.5A
4
RRYY
RRYy
ACTUAL
RESULTS
SUPPORT
HYPOTHESIS
1/
RrYY
RrYy
Actual results
contradict
hypothesis
RY
rY
Ry
ry
4
RrYy
RrYy
RRyy
Rryy
rryy
Ry
1/
RrYy
rrYy
Rryy
4
4
ry
9/
16
3/
16
3/
16
1/
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
The principle of independent assortment is revealed
by tracking two characteristics at once
• By looking at two characteristics at once,
Mendel found that the genes of a pair segregate
independently of other gene pairs during
gamete formation
– This is known as the principle of independent
assortment
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• Independent assortment of two genes in the
Labrador retriever
Blind
PHENOTYPES
GENOTYPES
Black coat,
normal vision
B_N_
Black coat,
blind (PRA)
B_nn
MATING OF HETEROZYOTES
(black, normal vision)
PHENOTYPIC RATIO
OF OFFSPRING
9 black coat,
normal vision
BbNn
3 black coat,
blind (PRA)
Figure 9.5B
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Blind
Chocolate coat,
normal vision
bbN_
Chocolate coat,
blind (PRA)
bbnn
BbNn
3 chocolate coat,
normal vision
1 chocolate coat,
blind (PRA)
Principle of Independent
Assortment
Dihybrid cross
An experimental mating of individuals in which
the inheritance of 2 traits is tracked.
When the inheritance of 2 traits is tracked in an
individual, the dominant/recessive traits does
not always appear together.
The individual may be dominant in one of the
traits and recessive in the other.
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Principle of Independent Assortment
• Genes for different characteristics are not
connected and each pair of genes for a
characteristic separate independently during
meiosis.
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Solving Dihybrid Problems
1. List the genotypes of each parent.
2. Make all possible combinations of the
gametes
3. Construct a 16 square Punnett square.
4. List the possible genotypes of the offspring
and determine the genotypic ratio.
5. List the possible phenotypes of the offspring
and determine the phenotypic ratio.
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Solving Dihybrid Problems
• Example: In humans freckles (F) is dominant
and no freckles (f) is recessive. Normal arches
(A) are dominant and flat feet (a) is recessive.
A man who has freckles and flat feet (FFaa)
marries a woman without freckles and normal
arches (ffAA). What are the possible genotypes
and phenotypes of their children?
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Connection: Genetic traits in humans can be
tracked through family pedigrees
• The inheritance of many
human traits follows
Mendel’s principles and
the rules of probability
Figure 9.8A
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• Family pedigrees are used to determine
patterns of inheritance and individual
genotypes
Dd
Joshua
Lambert
Dd
Abigail
Linnell
D_?
Abigail
Lambert
D_?
John
Eddy
dd
Jonathan
Lambert
Dd
Dd
dd
D_?
Hepzibah
Daggett
Dd
Elizabeth
Eddy
Dd
Dd
Dd
dd
Female Male
Deaf
Figure 9.8B
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Hearing
Connection: Many inherited disorders in humans
are controlled by a single gene
• Most such
disorders are
caused by
autosomal
recessive alleles
– Examples:
cystic fibrosis,
sickle-cell
disease
Normal
Dd
PARENTS
Normal
Dd
D
D
Eggs
Sperm
DD
Normal
d
OFFSPRING
d
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Figure 9.9A
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Inherited Single Gene Disorders
Recessive disorders
• Most single gene disorders
• Relatively harmless disorders to deadly
diseases
• Most born to normal parents who are carriers
– Carrier – an individual who is heterozygous for
a recessive disorder and does not show
symptoms of the disorder
• Carriers have a 1 in 4 chance of having a child
with a recessive disorder
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• A few are caused by dominant alleles
– Examples: achondroplasia, Huntington’s disease
Figure 9.9B
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Table 9.9
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Connection: Fetal testing can spot many inherited
disorders early in pregnancy
• Karyotyping and biochemical tests of fetal cells
and molecules can help people make
reproductive decisions
– Fetal cells can be obtained through
amniocentesis
Amniotic
fluid
Amniotic
fluid
withdrawn
Centrifugation
Fluid
Fetal
cells
Fetus
(14-20
weeks)
Biochemical
tests
Placenta
Figure 9.10A
Uterus
Cervix
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Several
weeks later
Cell culture
Karyotyping
• Chorionic villus sampling is another procedure
that obtains fetal cells for karyotyping
Fetus
(10-12
weeks)
Several hours
later
Placenta
Suction
Chorionic villi
Fetal cells
(from chorionic villi)
Karyotyping
Some
biochemical
tests
Figure 9.10B
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• Examination of the fetus with ultrasound is
another helpful technique
Figure 9.10C, D
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VARIATIONS ON MENDEL’S PRINCIPLES
The relationship of genotype to phenotype is rarely
simple
• Mendel’s principles are valid for all sexually
reproducing species
– However, often the genotype does not dictate the
phenotype in the simple way his principles
describe
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Incomplete dominance results in intermediate
phenotypes
• When an offspring’s
phenotype—such
as flower color— is
in between the
phenotypes of its
parents, it exhibits
incomplete
dominance
P GENERATION
White
rr
Red
RR
Gametes
R
r
Pink
Rr
F1 GENERATION
1/
1/
Eggs
1/
F2 GENERATION
2
2
2
R
1/
2
r
1/
R
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R
Red
RR
r
Pink
Rr
Sperm
1/
Pink
rR
White
rr
Figure 9.12A
2
2
r
• 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
Figure 9.12B
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Severe disease
Incomplete Dominance
• In a cross between a homozygous dominant
parent and a homozygous recessive parent the
phenotype of the offspring is in between the
phenotypes of the parents.
• Example: When red snapdragons are crossed
with white snapdragons all the offspring have
pink flowers
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– The alleles for A and B blood types are
codominant, and both are expressed in the
phenotype
Blood
Group
(Phenotype)
Genotypes
Antibodies
Present in
Blood
Reaction When Blood from Groups Below Is Mixed with
Antibodies from Groups at Left
O
O
ii
Anti-A
Anti-B
A
IA IA
or
IA i
Anti-B
B
IB IB
or
IB i
Anti-A
AB
IA IB
Figure 9.13
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A
B
AB
Many genes have more than two alleles in the
population
Multiple allele traits
• 3 or more alleles of the same gene code for a
single trait
• Example: the three alleles (IA, IB, i) for ABO
blood type in humans
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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
Pain and
fever
Paralysis
Brain
damage
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
Figure 9.14
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A single gene may affect many phenotypic
characteristics
Pleoitropy
• A single gene may affect phenotype in many
ways
• Example: the allele for sickle-cell disease
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P GENERATION
aabbcc
AABBCC
(very light) (very dark)
F1 GENERATION
Eggs
Sperm
Fraction of population
AaBbCc AaBbCc
Skin pigmentation
F2 GENERATION
Figure 9.16
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A single characteristic may be influenced by many
genes
Polygenic traits
• Trait that is controlled by 2 or more genes.
• This situation creates a continuum of
phenotypes
– When the range of traits is graphed a bell
shaped curve is seen
• Example: skin color, eye color
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Match the description with its pattern of
inheritance
1. There are 3 different alleles for a blood group,
IA, IB, and i, but an individual has only two at
a time.
2. The sickle cell allele, s, is responsible for a
variety of phenotypic effects, from pain and
fever to damage to the heart, lungs, joints,
brain or kidneys.
3. If a red shorthorn cow is mated with a white
bull, all their offspring are roan, a phenotype
that has a mixture of red and white hairs.
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4. Independent genes at 4 different loci are
responsible for determining a person’s HLA
tissue type, important in organ transplants
and certain diseases.
5. When graphed, the number of individuals of
various heights forms a bell shaped curve.
6. Chickens homozygous for the black allele are
black, and chickens homozygous for the white
allele are white. Heterozygous chickens are
gray.
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• The chromosomal basis of Mendel’s principles
Figure 9.17
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THE CHROMOSOMAL BASIS OF
INHERITANCE
Chromosome behavior accounts for Mendel’s
principles
• Genes are located on chromosomes
– Their behavior during meiosis accounts for
inheritance patterns
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Figure 9.18
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Genes on the same chromosome tend to be
inherited together
Linked genes
• Genes that are located close together on the
same chromosome tend to be inherited together
• These genes usually do not follow Mendel’s
principle of independent assortment
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(male)
(female)
Parents’
diploid
cells
X
Y
Male
Sperm
Egg
Offspring
(diploid)
Figure 9.21A
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SEX CHROMOSOMES AND SEX-LINKED
GENES
Chromosomes determine sex in many species
• Many animals including humans have a pair of
sex chromosomes
• A human male has one X chromosome and one
Y chromosome
• A human female has two X chromosomes
• Whether a sperm cell has an X or Y
chromosome determines the sex of the
offspring
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• Other systems of sex determination exist in
other animals and plants
– The X-O system
– The Z-W system
– Chromosome number
Figure 9.21B-D
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Sex Chromosomes and Sex-Linked Genes
• The genetic basis of sex determination isn’t
fully understood:
– Gene SRY on the Y chromosome plays a crucial
role
– SRY triggers testis development
– Absence of SRY results in overy development
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Sex-linked genes exhibit a unique pattern of
inheritance
• All genes on the sex chromosomes are said to be
sex-linked
– In many organisms, the X chromosome carries
many genes unrelated to sex
– Fruit fly eye
color is a
sex-linked
characteristic
Figure 9.22A
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– Their inheritance pattern reflects the fact that
males have one X chromosome and females
have two
– These figures illustrate inheritance patterns for
white eye color (r) in the fruit fly, an X-linked
recessive trait
Female
XRXR
Male
Xr Y
XR
Female
XRXr
Xr
XRXr
Male
XRY
XRY
Xr
XRXR
XrXR
XRY
XrY
R = red-eye allele
r = white-eye allele
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Male
XRXr
XR
XR
Y
Female
XrY
Xr
XR
Y
Xr
XRXr
Xr Xr
Y
XRY
XrY
Figure 9.22B-D
Connection: Sex-linked disorders affect mostly
males
• Most sex-linked human
disorders are due to
recessive alleles
– Examples: hemophilia,
red-green color blindness
– These are mostly seen in males
Figure 9.23A
– A male receives a single X-linked allele from his
mother, and will have the disorder, while a
female has to receive the allele from both
parents to be affected
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• A high incidence of hemophilia has plagued the
royal families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.23B
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Sex-Linked Disorders
• Other sex-linked disorders are
– Duchenne muscular dystrophy – weakening and
loss of muscle tissue
– Fragile X syndrome – abnormal X chromosome,
most common cause of mental retardation in
boys
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Solving Sex-Linked Problems
• Example One: Eye color is a sex-linked trait in
fruit flies and is carried on the X chromosome.
Red eye color (R) is dominant over white eye
color (r). What is the sex and eye color of the
offsrping of a homozygous red eyed female and
a white eyed male?
• Example Two: What is the sex and eye color of
the offspring of a heterozygous red eyed female
fruit fly and a red eyed male fruit fly?
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