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CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
15
The
Chromosomal
Basis of
Inheritance
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
Locating Genes Along Chromosomes
 Mendel’s “hereditary factors” were purely abstract
when first proposed
 Today we can show that the factors—genes—are
located on chromosomes
 The location of a particular gene can be seen by
tagging isolated chromosomes with a fluorescent
dye that highlights the gene
© 2014 Pearson Education, Inc.
Figure 15.1
© 2014 Pearson Education, Inc.
Figure 15.1a
© 2014 Pearson Education, Inc.
 Cytologists worked out the process of mitosis in
1875, using improved techniques of microscopy
 Biologists began to see parallels between the
behavior of Mendel’s proposed hereditary factors
and chromosomes
 Around 1902, Sutton and Boveri and others
independently noted the parallels and the
chromosome theory of inheritance began
to form
© 2014 Pearson Education, Inc.
Figure 15.2
Yellow-round
seeds
(YYRR)
P Generation
Y
r
R R
Y
Green-wrinkled
seeds (yyrr)
y
r
y
Meiosis
Fertilization
y
R Y
Gametes
F1 Generation
R
All F1 plants produce
yellow-round seeds (YyRr).
R
y
r
y
r
Y
r
Y
LAW OF INDEPENDENT
ASSORTMENT Alleles of
genes on nonhomologous
chromosomes assort
independently.
Meiosis
LAW OF
SEGREGATION
The two alleles for each
gene separate.
R
r
Y
y
Metaphase
I
r
R
Y
y
1
1
R
r
Y
y
r
R
Y
y
Anaphase I
R
r
Y
y
2
y
Y
Y
R
R
1
4
YR
F2 Generation
Metaphase
II
4
© 2014 Pearson Education, Inc.
y
2
Y
r
1
yr
4
Yr
An F1 × F1 cross-fertilization
3 Fertilization
recombines the
R and r alleles at random.
Y
r
r
1
R
Y
y
r
r
9
:3
:3
:1
y
y
R
R
1
4
yR
3 Fertilization results
in the 9:3:3:1
phenotypic ratio in
the F2 generation.
Figure 15.2a
P Generation
Yellow-round
seeds (YYRR)
Y
Y
r
R R
y
Green-wrinkled
seeds (yyrr)
y
r
Meiosis
Fertilization
Gametes
© 2014 Pearson Education, Inc.
R Y
y
r
Figure 15.2b
F1 Generation
R
r
y
Y
LAW OF
SEGREGATION
The two alleles for
each gene
separate.
All F1 plants produce
yellow-round seeds (YyRr).
R
r
Y
y
LAW OF INDEPENDENT
ASSORTMENT Alleles
of genes on
nonhomologous
chromosomes assort
independently.
Meiosis
r
R
Y
y
r
R
Y
Metaphase
I
y
1
1
R
r
r
R
Y
y
Anaphase I
Y
y
r
R
Metaphase
II
r
R
Y
y
2
2
y
Y
y
Y
Y
R
R
1
4
YR
© 2014 Pearson Education, Inc.
r
1
r
4
yr
Y
Y
y
r
r
1
4
Yr
y
y
R
1
R
4
yR
Figure 15.2c
LAW OF
SEGREGATION
F2 Generation
3 Fertilization
An F1 × F1 cross-fertilization
recombines the
R and r alleles
:3
9
:3 :1
at random.
© 2014 Pearson Education, Inc.
LAW OF
INDEPENDENT
ASSORTMENT
3 Fertilization results
in the 9:3:3:1
phenotypic ratio in
the F2 generation.
Concept 15.1: Morgan showed that Mendelian
inheritance has its physical basis in the
behavior of chromosomes: Scientific inquiry
 The first solid evidence associating a specific gene
with a specific chromosome came in the early 20th
century from the work of Thomas Hunt Morgan
 These early experiments provided convincing
evidence that the chromosomes are the location
of Mendel’s heritable factors
© 2014 Pearson Education, Inc.
Morgan’s Choice of Experimental Organism
 For his work, Morgan chose to study Drosophila
melanogaster, a common species of fruit fly
 Several characteristics make fruit flies a
convenient organism for genetic studies
 They produce many offspring
 A generation can be bred every two weeks
 They have only four pairs of chromosomes
© 2014 Pearson Education, Inc.
 Morgan noted wild type, or normal, phenotypes
that were common in the fly populations
 Traits alternative to the wild type are called mutant
phenotypes
© 2014 Pearson Education, Inc.
Figure 15.3
© 2014 Pearson Education, Inc.
Figure 15.3a
© 2014 Pearson Education, Inc.
Figure 15.3b
© 2014 Pearson Education, Inc.
Correlating Behavior of a Gene’s Alleles with
Behavior of a Chromosome Pair
 In one experiment, Morgan mated male flies with
white eyes (mutant) with female flies with red eyes
(wild type)
 The F1 generation all had red eyes
 The F2 generation showed a 3:1 red to white eye
ratio, but only males had white eyes
 Morgan determined that the white-eyed mutant
allele must be located on the X chromosome
 Morgan’s finding supported the chromosome
theory of inheritance
© 2014 Pearson Education, Inc.
Figure 15.4
Experiment
Conclusion
P
Generation
P
Generation
F1
Generation
Results
F2
Generation
X
X
w+
w+
All offspring
had red eyes.
w
Eggs
F1
Generation
Sperm
w+
w+
Eggs
F2
Generation
w+
w
w+
Sperm
w+
w+
w+
w+
w
w
w+
© 2014 Pearson Education, Inc.
w
X
Y
w
Figure 15.4a
Experiment
P
Generation
F1
Generation
Results
F2
Generation
© 2014 Pearson Education, Inc.
All offspring
had red eyes.
Figure 15.4b
Conclusion
P
Generation
X
X
w+
X
Y
w+
w
Sperm
Eggs
F1
Generation
w+
w+
w+
w
w+
Eggs
F2
Generation
w+
Sperm
w+
w+
w+
w
w
w+
© 2014 Pearson Education, Inc.
w
w
Concept 15.2: Sex-linked genes exhibit unique
patterns of inheritance
 Morgan’s discovery of a trait that correlated with
the sex of flies was key to the development of the
chromosome theory of inheritance
 In humans and some other animals, there is a
chromosomal basis of sex determination
© 2014 Pearson Education, Inc.
The Chromosomal Basis of Sex
 In humans and other mammals, there are two
varieties of sex chromosomes: a larger X
chromosome and a smaller Y chromosome
 A person with two X chromosomes develops as a
female, while a male develops from a zygote with
one X and one Y
 Only the ends of the Y chromosome have regions
that are homologous with corresponding regions
of the X chromosome
© 2014 Pearson Education, Inc.
Figure 15.5
X
Y
© 2014 Pearson Education, Inc.
Figure 15.6
44 +
XY
44 +
XX
Parents
22 +
X
22 +
X
+
or 22
Y
Sperm
44 +
XX
Egg
or
44 +
XY
Zygotes (offspring)
(a) The X-Y system
22 +
XX
(b) The X-0 system
© 2014 Pearson Education, Inc.
22 +
X
76 +
ZW
76 +
ZZ
(c) The Z-W system
32
(Diploid)
16
(Haploid)
(d) The haplo-diploid system
Figure 15.6a
44 +
XY
44 +
XX
Parents
22 +
X
+
or 22
Y
Sperm
22 +
X
44 +
XX
Egg
or
44 +
XY
Zygotes (offspring)
(a) The X-Y system
22 +
XX
(b) The X-0 system
© 2014 Pearson Education, Inc.
22 +
X
Figure 15.6b
76 +
ZW
76 +
ZZ
(c) The Z-W system
32
(Diploid)
16
(Haploid)
(d) The haplo-diploid system
© 2014 Pearson Education, Inc.
 Short segments at the ends of the Y
chromosomes are homologous with the X,
allowing the two to behave like homologues during
meiosis in males
 A gene on the Y chromosome called SRY (sexdetermining region on the Y) is responsible for
development of the testes in an embryo
© 2014 Pearson Education, Inc.
 A gene that is located on either sex chromosome
is called a sex-linked gene
 Genes on the Y chromosome are called Y-linked
genes; there are few of these
 Genes on the X chromosome are called X-linked
genes
© 2014 Pearson Education, Inc.
Inheritance of X-Linked Genes
 X chromosomes have genes for many characters
unrelated to sex, whereas most Y-linked genes
are related to sex determination
© 2014 Pearson Education, Inc.
 X-linked genes follow specific patterns of
inheritance
 For a recessive X-linked trait to be expressed
 A female needs two copies of the allele
(homozygous)
 A male needs only one copy of the allele
(hemizygous)
 X-linked recessive disorders are much more
common in males than in females
© 2014 Pearson Education, Inc.
Figure 15.7
XNXN
Xn
XnY
Y
Eggs XN
XNXn XNY
XN
XNXn XNY
Sperm
(a)
XNXn
XN
Y
Eggs XN XNXN XNY
Xn
(b)
© 2014 Pearson Education, Inc.
XNXn
XNY
Sperm
Xn
XnY
Y
Eggs XN XNXn XNY
XNXn XnY
Xn
(c)
XnXn XnY
Sperm
 Some disorders caused by recessive alleles on
the X chromosome in humans
 Color blindness (mostly X-linked)
 Duchenne muscular dystrophy
 Hemophilia
© 2014 Pearson Education, Inc.
X Inactivation in Female Mammals
 In mammalian females, one of the two X
chromosomes in each cell is randomly inactivated
during embryonic development
 The inactive X condenses into a Barr body
 If a female is heterozygous for a particular gene
located on the X chromosome, she will be a
mosaic for that character
© 2014 Pearson Education, Inc.
Figure 15.8
X chromosomes
Allele for
orange fur
Early embryo:
Two cell
populations
in adult cat:
Active X
Allele for
black fur
Cell division and
X chromosome
inactivation
Black fur
© 2014 Pearson Education, Inc.
Active X
Inactive
X
Orange fur
Figure 15.8a
© 2014 Pearson Education, Inc.
Concept 15.3: Linked genes tend to be inherited
together because they are located near each
other on the same chromosome
 Each chromosome has hundreds or thousands of
genes (except the Y chromosome)
 Genes located on the same chromosome that tend
to be inherited together are called linked genes
© 2014 Pearson Education, Inc.
How Linkage Affects Inheritance
 Morgan did other experiments with fruit flies to see
how linkage affects inheritance of two characters
 Morgan crossed flies that differed in traits of body
color and wing size
© 2014 Pearson Education, Inc.
Figure 15.9
Experiment
P Generation
(homozygous)
Wild type (gray
body, normal wings)
Double mutant
(black body, vestigial wings)
b+ b+ vg+ vg+
b b vg vg
F1 dihybrid testcross
Wild-type F1 dihybrid
(gray body, normal wings)
Homozygous
recessive (black
body, vestigial wings)
b+ b vg+ vg
b b vg vg
Testcross
offspring
Eggs b+ vg+
b vg
Wild type
Black(gray-normal) vestigial
b+ vg
b vg+
Grayvestigial
Blacknormal
b vg
Sperm
b+ b vg+ vg
PREDICTED RATIOS
Genes on different
chromosomes:
Genes on the same
chromosome:
Results
© 2014 Pearson Education, Inc.
b b vg vg b+ b vg vg b b vg+ vg
1
:
1
:
1
:
1
1
:
1
:
0
:
0
965
:
944
:
206
:
185
Figure 15.9a
Experiment
P Generation
(homozygous)
Wild type (gray
body, normal
wings)
b+ b+ vg+ vg+
Double mutant
(black body, vestigial
wings)
b b vg vg
F1 dihybrid testcross
Wild-type F1 dihybrid
(gray body, normal
wings)
Homozygous
recessive (black
body, vestigial
wings)
b+ b vg+ vg
b b vg vg
© 2014 Pearson Education, Inc.
Figure 15.9b
Experiment
Testcross
offspring
Eggs
b+ vg+
b vg
BlackWild type
(gray-normal) vestigial
b+ vg
b vg+
Grayvestigial
Blacknormal
b vg
Sperm
b+ b vg+ vg
PREDICTED RATIOS
Genes on different
chromosomes:
Genes on the same
chromosome:
Results
© 2014 Pearson Education, Inc.
b b vg vg b+ b vg vg b b vg+ vg
1
:
1
:
1
:
1
1
:
1
:
0
:
0
965
:
944
:
206
:
185
 Morgan found that body color and wing size are
usually inherited together in specific combinations
(parental phenotypes)
 He noted that these genes do not assort
independently, and reasoned that they were on
the same chromosome
© 2014 Pearson Education, Inc.
Figure 15.UN01
F1 dihybrid female
and homozygous
recessive male
in testcross
b+ vg+
b vg
b vg
b vg
b+ vg+
b vg
Most offspring
or
b vg
© 2014 Pearson Education, Inc.
b vg
 However, nonparental phenotypes were also
produced
 Understanding this result involves exploring
genetic recombination, the production of
offspring with combinations of traits differing from
either parent
© 2014 Pearson Education, Inc.
Genetic Recombination and Linkage
 The genetic findings of Mendel and Morgan relate
to the chromosomal basis of recombination
© 2014 Pearson Education, Inc.
Recombination of Unlinked Genes: Independent
Assortment of Chromosomes
 Offspring with a phenotype matching one of the
parental phenotypes are called parental types
 Offspring with nonparental phenotypes (new
combinations of traits) are called recombinant
types, or recombinants
 A 50% frequency of recombination is observed for
any two genes on different chromosomes
© 2014 Pearson Education, Inc.
Figure 15.UN02
Gametes from yellow-round
dihybrid parent (YyRr)
Gametes from
testcross
homozygous
recessive
parent (yyrr)
YR
yr
Yr
yR
YyRr
yyrr
Yyrr
yyRr
yr
Parentaltype
offspring
© 2014 Pearson Education, Inc.
Recombinant
offspring
Recombination of Linked Genes: Crossing Over
 Morgan discovered that genes can be linked, but
the linkage was incomplete, because some
recombinant phenotypes were observed
 He proposed that some process must occasionally
break the physical connection between genes on
the same chromosome
 That mechanism was the crossing over of
homologous chromosomes
© 2014 Pearson Education, Inc.
Figure 15.10
F1 dihybrid
testcross
Double mutant (black body,
vestigial wings)
Wild type (gray body,
normal wings)
P generation
(homozygous)
b+ vg+
b vg
b+ vg+
b vg
Wild-type F1
dihybrid (gray body,
normal wings)
Replication of
chromosomes
Homozygous recessive
(black body,
vestigial wings)
b+ vg+
b vg
b vg
b vg
Replication of
chromosomes
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
Meiosis I
b+ vg+
Meiosis I and II
b+ vg
b vg+
b vg
Meiosis II
b vg
b+ vg
944
965
BlackWild type
(gray-normal) vestigial
206
Grayvestigial
Eggs
Testcross
offspring
b+ vg+
Recombinant
chromosomes
b vg+
185
Blacknormal
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
b vg
Sperm
Recombinant
Parental-type
offspring
offspring
391 recombinants
Recombination
=
× 100 = 17%
frequency
2,300 total offspring
© 2014 Pearson Education, Inc.
Figure 15.10a
P generation (homozygous)
Wild type (gray body,
normal wings)
b+ vg+
b vg
b+ vg+
b vg
Wild-type F1
dihybrid (gray body,
normal wings)
b+ vg+
b vg
© 2014 Pearson Education, Inc.
Double mutant (black body,
vestigial wings)
Figure 15.10b
F1 dihybrid testcross +
b vg+
Wild-type F1
dihybrid
b vg
(gray body,
normal wings)
Meiosis I
b vg
Homozygous
recessive
b vg (black body,
vestigial wings)
b+ vg+
b vg
b+ vg+
b vg
b vg
b vg
b vg
b vg
b+ vg+
b+ vg
b vg+
Meiosis I and II
b vg Recombinant
Meiosis II
Eggs
b+vg+
chromosomes
b vg
b+ vg
b vg+
b vg
Sperm
© 2014 Pearson Education, Inc.
Figure 15.10c
Recombinant
chromosomes
Meiosis II
b vg
b+ vg
b vg+
965
944
Wild type
Black(gray-normal) vestigial
206
Grayvestigial
185
Blacknormal
Eggs
Testcross
offspring
b+vg+
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
b vg
Sperm
Recombinant
Parental-type
offspring
offspring
391 recombinants
Recombination
=
× 100 = 17%
frequency
2,300 total offspring
© 2014 Pearson Education, Inc.
Animation: Crossing Over
© 2014 Pearson Education, Inc.
New Combinations of Alleles: Variation for
Natural Selection
 Recombinant chromosomes bring alleles together
in new combinations in gametes
 Random fertilization increases even further the
number of variant combinations that can be
produced
 This abundance of genetic variation is the raw
material upon which natural selection works
© 2014 Pearson Education, Inc.
Mapping the Distance Between Genes Using
Recombination Data: Scientific Inquiry
 Alfred Sturtevant, one of Morgan’s students,
constructed a genetic map, an ordered list of
the genetic loci along a particular chromosome
 Sturtevant predicted that the farther apart two
genes are, the higher the probability that a
crossover will occur between them and therefore
the higher the recombination frequency
© 2014 Pearson Education, Inc.
 A linkage map is a genetic map of a chromosome
based on recombination frequencies
 Distances between genes can be expressed as
map units; one map unit, or centimorgan,
represents a 1% recombination frequency
 Map units indicate relative distance and order,
not precise locations of genes
© 2014 Pearson Education, Inc.
Figure 15.11
Results
Recombination
frequencies
9%
Chromosome
17%
b
© 2014 Pearson Education, Inc.
9.5%
cn
vg
 Genes that are far apart on the same chromosome
can have a recombination frequency near 50%
 Such genes are physically linked, but genetically
unlinked, and behave as if found on different
chromosomes
© 2014 Pearson Education, Inc.
 Sturtevant used recombination frequencies to
make linkage maps of fruit fly genes
 He and his colleagues found that the genes
clustered into four groups of linked genes (linkage
groups)
 The linkage maps, combined with the fact that
there are four chromosomes in Drosophila,
provided additional evidence that genes are
located on chromosomes
© 2014 Pearson Education, Inc.
Figure 15.12
Mutant phenotypes
Short
aristae
0
Maroon
eyes
48.5
16.5
Long
Red
aristae
eyes
(appendages
on head)
© 2014 Pearson Education, Inc.
Black Cinnabar Vestigial Down- Brown
wings curved eyes
eyes
body
wings
Gray
body
57.5
Red
eyes
67.0 75.5
104.5
Normal Normal Red
wings wings eyes
Wild-type phenotypes
Concept 15.4: Alterations of chromosome
number or structure cause some genetic
disorders
 Large-scale chromosomal alterations in humans
and other mammals often lead to spontaneous
abortions (miscarriages) or cause a variety of
developmental disorders
 Plants tolerate such genetic changes better than
animals do
© 2014 Pearson Education, Inc.
Abnormal Chromosome Number
 In nondisjunction, pairs of homologous
chromosomes do not separate normally during
meiosis
 As a result, one gamete receives two of the same
type of chromosome, and another gamete
receives no copy
© 2014 Pearson Education, Inc.
Figure 15.13-1
Meiosis I
Nondisjunction
© 2014 Pearson Education, Inc.
Figure 15.13-2
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
© 2014 Pearson Education, Inc.
Figure 15.13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1 n−1
n−1
n+1
n−1
n
n
Number of chromosomes
(a) Nondisjunction of homologous chromosomes in
meiosis I
© 2014 Pearson Education, Inc.
(b) Nondisjunction of sister
chromatids in meiosis II
Video: Nondisjunction in Mitosis
© 2014 Pearson Education, Inc.
 Aneuploidy results from the fertilization of
gametes in which nondisjunction occurred
 Offspring with this condition have an abnormal
number of a particular chromosome
© 2014 Pearson Education, Inc.
 A monosomic zygote has only one copy of a
particular chromosome
 A trisomic zygote has three copies of a particular
chromosome
© 2014 Pearson Education, Inc.
 Polyploidy is a condition in which an organism
has more than two complete sets of chromosomes
 Triploidy (3n) is three sets of chromosomes
 Tetraploidy (4n) is four sets of chromosomes
 Polyploidy is common in plants, but not animals
 Polyploids are more normal in appearance than
aneuploids
© 2014 Pearson Education, Inc.
Alterations of Chromosome Structure
 Breakage of a chromosome can lead to four types
of changes in chromosome structure
 Deletion removes a chromosomal segment
 Duplication repeats a segment
 Inversion reverses orientation of a segment within
a chromosome
 Translocation moves a segment from one
chromosome to another
© 2014 Pearson Education, Inc.
Figure 15.14
(c) Inversion
(a) Deletion
A
B C D
E
F G
A
H
B C D
A deletion removes a
chromosomal segment.
A
B C
E
F G
H
F
A
G H
B C B
E
F G
H
C D
E
F G
E
F G
H
M N
O P
Q
R
A translocation moves a
segment from one chromosome
to a nonhomologous
chromosome.
H
M N
© 2014 Pearson Education, Inc.
C B
B C D
A duplication repeats
a segment.
A
H
(d) Translocation
(b) Duplication
B C D E
F G
An inversion reverses a
segment within a chromosome.
A D
A
E
O C D
E
F G
H
A
B P
Q
R
Figure 15.14a
(a) Deletion
A
B C D
E
F G H
A deletion removes a
chromosomal segment.
A
B C
E
F
G H
(b) Duplication
A
B C D E
F
G H
A duplication repeats
a segment.
A B
© 2014 Pearson Education, Inc.
C B C D
E
F G H
Figure 15.14b
(c) Inversion
A B
C D E
F G
H
An inversion reverses a
segment within a chromosome.
A D C
B E
F G
H
(d) Translocation
A B
C D E
F G
H
M N O P
Q
R
A translocation moves a
segment from one chromosome
to a nonhomologous
chromosome.
M N O C
© 2014 Pearson Education, Inc.
D
E
F G H
A
B P
Q
R
Human Disorders Due to Chromosomal
Alterations
 Alterations of chromosome number and structure
are associated with some serious disorders
 Some types of aneuploidy appear to upset the
genetic balance less than others, resulting in
individuals surviving to birth and beyond
 These surviving individuals have a set of
symptoms, or syndrome, characteristic of the type
of aneuploidy
© 2014 Pearson Education, Inc.
Down Syndrome (Trisomy 21)
 Down syndrome is an aneuploid condition that
results from three copies of chromosome 21
 It affects about one out of every 700 children born
in the United States
 The frequency of Down syndrome increases with
the age of the mother, a correlation that has not
been explained
© 2014 Pearson Education, Inc.
Figure 15.15
© 2014 Pearson Education, Inc.
Figure 15.15a
© 2014 Pearson Education, Inc.
Figure 15.15b
© 2014 Pearson Education, Inc.
Aneuploidy of Sex Chromosomes
 Nondisjunction of sex chromosomes produces a
variety of aneuploid conditions
 XXX females are healthy, with no unusual physical
features
 Klinefelter syndrome is the result of an extra
chromosome in a male, producing XXY individuals
 Monosomy X, called Turner syndrome, produces
X0 females, who are sterile; it is the only known
viable monosomy in humans
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Disorders Caused by Structurally Altered
Chromosomes
 The syndrome cri du chat (“cry of the cat”), results
from a specific deletion in chromosome 5
 A child born with this syndrome is severely
intellectually disabled and has a catlike cry;
individuals usually die in infancy or early childhood
 Certain cancers, including chronic myelogenous
leukemia (CML), are caused by translocations
of chromosomes
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Figure 15.16
Normal chromosome 9
Normal chromosome 22
Reciprocal translocation
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
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Concept 15.5: Some inheritance patterns are
exceptions to standard Mendelian inheritance
 There are two normal exceptions to Mendelian
genetics
 One exception involves genes located in the
nucleus, and the other exception involves genes
located outside the nucleus
 In both cases, the sex of the parent contributing an
allele is a factor in the pattern of inheritance
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Genomic Imprinting
 For a few mammalian traits, the phenotype
depends on which parent passed along the alleles
for those traits
 Such variation in phenotype is called genomic
imprinting
 Genomic imprinting involves the silencing of
certain genes depending on which parent passes
them on
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Figure 15.17
Paternal
chromosome
Maternal
chromosome
Normal Igf2 allele
is expressed.
Normal Igf2 allele
is not expressed.
Normal-sized mouse
(wild type)
(a) Homozygote
Mutant Igf2 allele
inherited from mother
Normal-sized mouse (wild type)
Normal Igf2 allele
is expressed.
Mutant Igf2 allele
is not expressed.
(b) Heterozygotes
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Mutant Igf2 allele
inherited from father
Dwarf mouse (mutant)
Mutant Igf2 allele
is expressed.
Normal Igf2 allele
is not expressed.
 It appears that imprinting is the result of the
methylation (addition of —CH3) of cysteine
nucleotides
 Genomic imprinting is thought to affect only a
small fraction of mammalian genes
 Most imprinted genes are critical for embryonic
development
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Inheritance of Organelle Genes
 Extranuclear genes (or cytoplasmic genes) are
found in organelles in the cytoplasm
 Mitochondria, chloroplasts, and other plant
plastids carry small circular DNA molecules
 Extranuclear genes are inherited maternally
because the zygote’s cytoplasm comes from
the egg
 The first evidence of extranuclear genes came
from studies on the inheritance of yellow or white
patches on leaves of an otherwise green plant
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Figure 15.18
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 Some defects in mitochondrial genes prevent cells
from making enough ATP and result in diseases
that affect the muscular and nervous systems
 For example, mitochondrial myopathy and Leber’s
hereditary optic neuropathy
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Figure 15.UN03a
Offspring
from testcross of
AaBb (F1) ×
aabb
Purple
stem/short
petals
(A–B–)
Green
stem/short
petals
(aaB–)
Purple
stem/long
petals
(A–bb)
Green
stem/long
petals
(aabb)
Expected
ratio if the
genes are
unlinked
1
1
1
1
220
210
231
239
Expected
number of
offspring
(of 900)
Observed
number of
offspring
(of 900)
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Figure 15.UN03b
Testcross
Offspring
Expected
(e)
Observed
(o)
(A−B−)
220
(aaB−)
210
(A−bb)
231
(aabb)
239
Deviation
(o − e)
(o − e)2 (o − e)2/e
2 = Sum
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Figure 15.UN03c
Cosmos plants
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Figure 15.UN04
Sperm
P generation
gametes
D
C
B
A
c
b
a
d
E
The alleles of unlinked
genes are either on
separate chromosomes
(such as d and e)
or so far apart on the
same chromosome
(c and f ) that they
assort independently.
This F1 cell has 2n = 6 chromosomes and is heterozygous for all
six genes shown (AaBbCcDdEeFf ).
Red = maternal; blue = paternal.
D
e
C
B
A
F
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e
f
F
Each chromosome has
hundreds or thousands
of genes. Four (A, B, C,
F ) are shown on this
one.
Egg
d
E
cb
a
f
Genes on the same chromosome whose alleles are so
close together that they do
not assort independently
(such as a, b, and c) are said
to be genetically linked.
Figure 15.UN05
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