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Chapter 9 Patterns of Inheritance
MENDEL’S LAWS
© 2012 Pearson Education, Inc.
9.1 The science of genetics has ancient roots
 The idea that hereditary materials mix in forming
offspring, called the blending hypothesis, was
– suggested in the 19th century by scientists studying
plants but
– later rejected because it did not explain how traits that
disappear in one generation can reappear in later
generations.
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
9.2 Experimental genetics began in an abbey
garden
 In 1866, 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.
© 2012 Pearson Education, Inc.
Figure 9.2A
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.
© 2012 Pearson Education, Inc.
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)
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
9.3 Mendel’s law of segregation describes the
inheritance of a single character
 A cross between two individuals differing in a
single character is a monohybrid cross.
 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.
© 2012 Pearson Education, Inc.
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
 The all-purple F1 generation did not produce light
purple flowers, as predicted by the blending
hypothesis.
 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.
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
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.
– The phenotype is the appearance or expression of a trait.
– The genotype is the genetic makeup of a trait.
– The same phenotype may be determined by more than one
genotype.
© 2012 Pearson Education, Inc.
9.3 Mendel’s law of segregation describes the
inheritance of a single character
4. A sperm or egg carries only one allele for each inherited
character because allele pairs separate (segregate) from
each other during the production of gametes. This
statement is called the law of segregation.
 Mendel’s hypotheses also explain the 3:1 ratio in the
F2 generation.
– The F1 hybrids all have a Pp genotype.
– A Punnett square shows the four possible combinations
of alleles that could occur when these gametes combine.
© 2012 Pearson Education, Inc.
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
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.
© 2012 Pearson Education, Inc.
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
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.
 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
© 2012 Pearson Education, Inc.
Figure 9.5A
P generation RRYY
Gametes RY
F1 generation
rryy

ry
Sperm
RrYy
1
4
RY
1
4
rY
1
4
Ry
1
4
ry
Sperm
1
2
1
2
F2 generation
RY
1
2
ry
RY
Eggs
1
2
ry
1
4
RY
1
4
rY
Eggs
1
4
1
4
The hypothesis of dependent assortment
Data did not support; hypothesis refuted
RRYY RrYY
RRYy
RrYy
RrYY
RrYy
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.
© 2012 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.
© 2012 Pearson Education, Inc.
Figure 9.5B
Blind
Blind
Phenotypes
Genotypes
Black coat,
normal vision
B_N_
Black coat,
blind (PRA)
B_nn
Chocolate coat,
normal vision
bbN_
Chocolate coat,
blind (PRA)
bbnn
Mating of double heterozygotes (black coat, normal vision)
BbNn
BbNn

Blind
Blind
Phenotypic ratio
of the offspring
9
Black coat,
normal vision
3
Black coat,
blind (PRA)
3
Chocolate coat,
normal vision
1
Chocolate coat,
blind (PRA)
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.
 The following figure demonstrates how a testcross
can be performed to determine the genotype of a
Lab with normal eyes.
© 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.
© 2012 Pearson Education, Inc.
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 Genetic traits in humans can be tracked
through family pedigrees
 The inheritance of some human traits follows
Mendel’s laws.
 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 9.8
Straight hairline
Widow’s peak
Figure 9.8
KEY Female Male
Widow’s peak hairline trait
Straight hairline trait
H: widow’s peak allele
h: straight allele
Mating
1ST
GENERATION
Al
Beth
Charles
Children
Evelyn
Frank
Gary
Debbie
2ND
GENERATION
Henry
Straight hairline
Isabel
Widow’s peak
3RD
GENERATION
Kristin
Juliana
Lori
Figure 9.8
KEY Female Male
Widow’s peak hairline trait
Straight hairline trait
H: widow’s peak allele
h: straight allele
1ST
GENERATION
Al
Beth
Charles
Debbie
Hh
Hh
hh
Hh
2ND
GENERATION
Evelyn
Frank
Gary
Henry
hh
hh
Hh
Straight hairline
A parent with a widow’s
peak who has a child
with a straight hairline
must be Hh.
Isabel
Juliana
Hh
hh
3RD
GENERATION
Kristin
hh
Kristin has a straight
Lori
hairline but her parents
do not, so straight hairline
must be homozygous
recessive (hh).
Widow’s peak
Figure 9.8
KEY Female Male
Widow’s peak hairline trait
Straight hairline trait
H: widow’s peak allele
h: straight allele
1ST
GENERATION
Al
Beth
Charles
Debbie
Hh
Hh
hh
Hh
2ND
GENERATION
Evelyn
Frank
Gary
Henry
Isabel
Juliana
HH
hh
hh
Hh
Hh
hh
or
Hh
Straight hairline
Widow’s peak
3RD
GENERATION
Not all genotypes can be
determined. Lori could be
Kristin HH or Hh and there is no
Lori
way to know (unless she
hh has some children and the HH
or
pedigree is extended).
Hh
9.9 CONNECTION: Many inherited traits in
humans are controlled by a single gene
 Because a trait is dominant does not mean that it
is
– “normal” or
– more common than a recessive trait.
 Wild-type traits are
– those most often seen in nature and
– not necessarily specified by dominant alleles.
© 2015 Pearson Education, Inc.
Figure 9.9a-0
Dominant Traits Recessive Traits
Freckles
No freckles
Normal
pigmentation
Albinism
Key
Wild-type (more common) trait
Mutant (less common) trait
9.9 CONNECTION: Many inherited traits in
humans are controlled by a single gene
 The genetic disorders listed in Table 9.9 are
known to be inherited as dominant or recessive
traits controlled by a single gene.
 These human disorders therefore show simple
inheritance patterns like the traits Mendel studied
in pea plants.
 The genes discussed in this module are all located
on autosomes.
© 2015 Pearson Education, Inc.
Table 9.9
9.9 CONNECTION: Many inherited traits in
humans are controlled by a single gene
 Thousands of human genetic disorders—ranging
in severity from relatively mild, such as albinism, to
invariably fatal, such as cystic fibrosis—are
inherited as recessive traits.
 Most people who have recessive disorders are
born to normal parents who
– are both heterozygotes, carriers of the recessive allele
for the disorder, but
– are phenotypically normal.
© 2015 Pearson Education, Inc.
Figure 9.9b
Normal
Aa
Parents
Normal
Aa
A Sperm a
A
Offspring
AA
Normal
Aa
Normal
(carrier)
Aa
Normal
(carrier)
aa
Albinism
Eggs
a
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, mostly caucasians.
 Dominant human disorders include
– achondroplasia, resulting in dwarfism, and
– Huntington’s disease, a degenerative disorder of the
nervous system.
© 2012 Pearson Education, Inc.
Figure 9.9B Achondroplasia
9.9 CONNECTION: Many inherited traits in
humans are controlled by a single gene
 Until relatively recently, the onset of symptoms
was the only way to know if a person had inherited
the Huntington’s allele.
 A genetic test is now available that can detect the
presence of the Huntington’s allele in an
individual’s genome.
 This is one of several genetic tests currently
available.
© 2015 Pearson Education, Inc.
9.10 CONNECTION: New technologies can
provide insight into one’s genetic legacy
 New technologies offer ways to obtain genetic
information
– before conception,
– during pregnancy, and
– after birth.
 Genetic testing can identify potential parents who
are heterozygous carriers for certain diseases.
© 2012 Pearson Education, Inc.
9.10 CONNECTION: New technologies can
provide insight into one’s genetic legacy
 Several technologies can be used for detecting
genetic conditions in a fetus.
– Amniocentesis extracts samples of amniotic fluid
containing fetal cells and permits
– karyotyping and
– biochemical tests on cultured fetal cells to detect other
conditions, such as Tay-Sachs disease.
– Chorionic villus sampling removes a sample of
chorionic villus tissue from the placenta and permits
similar karyotyping and biochemical tests.
© 2012 Pearson Education, Inc.
Figure 9.10A
Amniocentesis
Amniotic fluid
extracted
Ultrasound
transducer
Fetus
Chorionic Villus Sampling (CVS)
Tissue extracted
from the
Ultrasound
chorionic villi
transducer
Fetus
Placenta
Chorionic
villi
Placenta
Uterus
Cervix
Cervix
Uterus
Centrifugation
Amniotic fluid
Fetal cells
Several
hours
Cultured
cells
Several
weeks
Several
weeks
Karyotyping
Biochemical
and genetics
tests
Fetal cells
Several
hours
Several
hours
9.10 CONNECTION: New technologies can
provide insight into one’s genetic legacy
 Blood tests on the mother at 15–20 weeks of
pregnancy can help identify fetuses at risk for
certain birth defects.
 Fetal imaging enables a physician to examine a
fetus directly for anatomical deformities. The most
common procedure is ultrasound imaging, using
sound waves to produce a picture of the fetus.
 Newborn screening can detect diseases that can
be prevented by special care and precautions.
© 2012 Pearson Education, Inc.
Figure 9.10B Ultrasound of a
fetus
9.10 CONNECTION: New technologies can
provide insight into one’s genetic legacy
 New technologies raise ethical considerations that
include
– the confidentiality and potential use of results of
genetic testing,
– time and financial costs, and
– determining what, if anything, should be done as a
result of the testing.
© 2012 Pearson Education, Inc.
VARIATIONS ON
MENDEL’S LAWS
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
Figure 9.11B
HH
Homozygous
for ability to make
LDL receptors
Genotypes
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
 Although an individual can at most carry two
different alleles for a particular gene, more than two
alleles often exist in the wider population.
 Human ABO blood group phenotypes involve three
alleles for a single gene.
 The four human blood groups, A, B, AB, and O,
result from combinations of these three alleles.
 The A and B alleles are both expressed in
heterozygous individuals, a condition known as
codominance.
© 2012 Pearson Education, Inc.
9.12 Many genes have more than two alleles in
the population
 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
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.
© 2012 Pearson Education, Inc.
Figure 9.13A Sickle cell
anemia
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
Damage to organs
Kidney failure
Heart failure
Spleen damage
Brain damage (impaired
mental function, paralysis)
Other effects
Pain and fever
Joint problems
Physical weakness
Anemia
Pneumonia and other
infections
9.14 A single character may be influenced by
many genes
 Many characteristics result from polygenic
inheritance, in which 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 9.14
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
9.15 The environment affects many characters
 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 9.15B
THE CHROMOSOMAL BASIS
OF INHERITANCE
© 2012 Pearson Education, Inc.
9.16 Chromosome behavior accounts for
Mendel’s laws
 The chromosome theory of inheritance states that
– genes occupy specific loci (positions) on chromosomes
and
– chromosomes undergo segregation and independent
assortment during meiosis.
© 2012 Pearson Education, Inc.
9.16 Chromosome behavior accounts for
Mendel’s laws
 Mendel’s laws correlate with chromosome
separation in meiosis.
– 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.
© 2012 Pearson Education, Inc.
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
9.17 SCIENTIFIC DISCOVERY: Genes on the same
chromosome tend to be inherited together
 Bateson and Punnett studied plants that did not show
a 9:3:3:1 ratio in the F2 generation. What they found
was an example of linked genes, which
– are located close together on the same chromosome and
– tend to be inherited together.
© 2012 Pearson Education, Inc.
Figure 9.17
The Experiment
Purple flower

PpLl
Phenotypes
PpLl
Long pollen
Observed
offspring
284
21
21
55
Purple long
Purple round
Red long
Red round
Prediction
(9:3:3:1)
215
71
71
24
The Explanation: Linked Genes
PL
Parental
diploid cell
PpLl
pl
Meiosis
Most
gametes
pl
PL
Fertilization
Sperm
PL
Most
offspring
pl
PL
PL
PL
pl
pl
pl
PL
pl
PL
Eggs
pl
3 purple long : 1 red round
Not accounted for: purple round and red long
9.18 Crossing over produces new combinations
of alleles
 Crossing over between homologous
chromosomes produces new combinations of
alleles in gametes.
 Linked alleles can be separated by crossing over,
forming recombinant gametes.
 The percentage of recombinants is the
recombination frequency.
© 2012 Pearson Education, Inc.
Figure 9.18A
p L
p l
PL
Parental gametes
pl
p L
Tetrad
Crossing over
(pair of
homologous
chromosomes)
P l
Recombinant gametes
Figure 9.18C
The Explanation
The Experiment
Gray body,
long wings
(wild type)
Black body,
vestigial wings
GgLl
ggll
Female
Male
GgLl
Female
GL
gl
gl
gl
ggll
Male
Crossing over
GL
Gl
g l
gl
gL
Eggs
Sperm
Offspring
Gray long
Black vestigial Gray vestigial
Black long
Offspring
GL
g l
G l
g L
g l
g l
g l
g l
Parental
965
944
Parental
phenotypes
Recombination frequency 
206
185
Recombinant
phenotypes
391 recombinants
 0.17 or 17%
2,300 total offspring
Recombinant
9.19 Geneticists use crossover data to map genes
 When examining recombinant frequency, Morgan
and his students found that the greater the
distance between two genes on a chromosome,
the more points there are between them where
crossing over can occur.
 Recombination frequencies can thus be used to
map the relative position of genes on
chromosomes.
© 2012 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.
Figure 9.20A Human sex
chromosomes
X
Y
Figure 9.20b
Parents
(diploid)
Gametes
(haploid)
Offspring
(diploid)
22
+
X
Male
Female
44
+
XY
44
+
XX
22
+
Y
Sperm
22
+
X
44
+
XX
Female
Egg
44
+
XY
Male
9.20 Chromosomes determine sex in many
species
 Grasshoppers, roaches, and some other insects
have an X-O system, in which
– O stands for the absence of a sex chromosome,
– females are XX, and
– males are XO.
 In certain fishes, butterflies, and birds,
– the sex chromosomes are Z and W,
– males are ZZ, and
– females are ZW.
© 2012 Pearson Education, Inc.
9.20 Chromosomes determine sex in many
species
 Some organisms lack sex chromosomes altogether.
 In bees, sex is determined by chromosome number.
– Females are diploid.
– Males are haploid.
© 2012 Pearson Education, Inc.
9.20 Chromosomes determine sex in many
species
 In some animals, environmental temperature
determines the sex.
– For some species of reptiles, the temperature at which
the eggs are incubated during a specific period of
development determines whether the embryo will develop
into a male or female.
– Global climate change may therefore impact the sex ratio
of such species.
© 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 9.21A Fruit fly eye
color determined by sexlinked 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.