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Key Concepts in Chapter 15
Chapter 15
1. Mendelian inheritance has its physical basis in the
behavior of chromosomes.
The Chromosomal Basis of
Inheritance
Dr. Wendy Sera
Houston Community
College
Biology 1406
2. Sex-linked genes exhibit unique patterns of inheritance.
3. Linked genes tend to be inherited together because
they are located near each other on the same
chromosome.
4. Alterations of chromosomes number or structure cause
some genetic disorders.
5. Some inheritance patterns are exceptions to standard
Mendelian inheritance.
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Locating Genes Along Chromosomes
Figure 15.1—Where are Mendel’s hereditary factors
located in the cell?
 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
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© 2014 Pearson Education, Inc.
Locating Genes Along Chromosomes, cont.
Locating Genes Along Chromosomes, cont.
 Cytologists had worked out the processes of
mitosis and meiosis by the late 1800s, using
improved techniques of microscopy
 The chromosome theory of inheritance
states:
 Biologists began to see parallels between the
behavior of Mendel’s proposed hereditary factors
and chromosomes
 Around 1902, Sutton, Boveri, and others
independently noted the parallels and the
chromosome theory of inheritance began
to form
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 Mendelian genes have specific loci (positions)
on chromosomes (singular = locus)
 Chromosomes undergo segregation and
independent assortment
 The behavior of chromosomes during meiosis
was said to account for Mendel’s laws of
segregation and independent assortment
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1
P Generation
Figure
15.2—The
chromosomal
basis of
Mendel’s
F Generation
laws
Yellow-round
seeds
(YYRR)
Y
r
R R
Y
Green-wrinkled
seeds (yyrr)
y
Concept 15.1: Morgan showed that Mendelian
inheritance has its physical basis in the
behavior of chromosomes: Scientific inquiry
r
y
Meiosis
Fertilization
y
R Y
Gametes
1
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
Metaphase
I
y
r
R
Y
y
1
1
R
r
R
Y
y
r
Anaphase I
Y
y
R
r
Y
y
2
R
R
1
4
YR
F2 Generation
2
r
1
yr
4
Yr
An F1 × F1 cross-fertilization
y
y
R
R
1
4
yR
3 Fertilization results
3 Fertilization
recombines the
R and r alleles at random.
9
:3
:3
 These early experiments provided convincing
evidence that the chromosomes are the location
of Mendel’s heritable factors
y
Y
r
r
4
R
Y
y
r
1
r
Y
y
Y
Y
Metaphase
II
 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
:1
in the 9:3:3:1
phenotypic ratio in
the F2 generation.
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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
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 Morgan noted wild type, or normal, phenotypes
that were common in the fly populations
 Traits alternative to the wild type are called
mutant phenotypes
Figure 15.3—
Morgan’s first
mutant
 They have only four pairs of chromosomes
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Correlating Behavior of a Gene’s Alleles with
Behavior of a Chromosome Pair
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Figure 15.4b—In a cross between a wild-type female fruit fly and a mutant white-eyed
male, what color eyes will the F1 and F2 offspring have?
P
Generation
 In one experiment, Morgan mated male flies with
white eyes (mutant) with female flies with red eyes
(wild type)
X
X
w+
w
Eggs
F1
Generation
 The F1 generation all had red eyes
w+
w
w+
Eggs
 Morgan determined that the white-eyed mutant
allele must be located on the X chromosome
F2
Generation
 Morgan’s finding supported the chromosome
theory of inheritance
w
Sperm
w+
w+
 The F2 generation showed a 3:1 red to white eye
ratio, but only males had white eyes
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X
Y
w+
w+
Sperm
w+
w+
w+
w
w
w
w+
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2
Concept 15.2: Sex-linked genes exhibit unique
patterns of inheritance
The Chromosomal Basis of Sex
 Morgan’s discovery of a trait that correlated with the sex of
flies was key to the development of the chromosome
theory of inheritance
 Only the ends of the Y chromosome have regions that are
homologous with corresponding regions of the X
chromosome, allowing the two to behave like homologues
during meiosis in males
 In humans and some other animals, there is a
chromosomal basis of sex determination: a larger X
chromosome and a smaller Y chromosome
 Each ovum contains an X chromosome, while a sperm
may contain either an X or a Y chromosome, thus the male
determines the sex of the offspring.
 A person with two X
chromosomes develops as a
female, while a male develops
from a zygote with one X and
one Y
 A gene on the Y chromosome called SRY (sexdetermining region on the Y) is responsible for
development of the testes in an embryo
X
Y
 Other animals have different methods of sex determination
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© 2014 Pearson Education, Inc.
Figure 15.6—Some chromosomal systems of sex determination
44 +
XY
22 +
X
44 +
XX
Parents
or
Sperm
44 +
XX
Egg
or
(b) The X-0 system
76 +
ZW
44 +
XY
Zygotes (offspring)
(a) The X-Y system
22 +
XX
 X chromosomes have genes for many characters
unrelated to sex, whereas most Y-linked genes
are related to sex determination
22 +
X
22 +
Y
Inheritance of X-Linked Genes
76 +
ZZ
(c) The Z-W system
32
(Diploid)
22 +
X
16
(Haploid)
(d) The haplo-diploid system
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Inheritance of X-Linked Genes, cont.
 X-linked genes follow specific patterns of
inheritance
 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
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XNXN
Figure 15.7—The
transmission of Xlinked recessive
traits
Xn
XNXn XNY
XN
XNXn XNY
Sperm
(a)
 A female needs two copies of the allele
(homozygous)
XNXn
 A male needs only one copy of the allele
(hemizygous)
Eggs
(b)
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Y
Eggs XN
 For a recessive X-linked trait to be expressed
 Thus, X-linked recessive disorders are much more
common in males than in females
XnY
XNXn
XNY
XN
Y
XN
XNXN
XNY
Xn
XNXn XnY
Sperm
Eggs
XnY
Xn
Y
XN
XNXn
XNY
Xn
XnXn XnY
Sperm
(c)
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3
Recessive X-Linked Disorders in Humans
 Some disorders caused by recessive alleles on
the X chromosome in humans are:
 Color blindness
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
 Duchenne muscular dystrophy
 If a female is heterozygous for a particular gene
located on the X chromosome, she will be a
mosaic for that character
 Hemophilia
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© 2014 Pearson Education, Inc.
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
Active X
Inactive
X
Black fur
Orange fur
Figure 15.8—X
inactivation and
the tortoiseshell
cat
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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
Brie Brie—A Handful of a Tortie!
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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
 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
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4
How Linkage Affects Inheritance, cont.
Genetic Recombination and Linkage
 However, nonparental phenotypes were also
produced
 The genetic findings of Mendel and Morgan relate
to the chromosomal basis of recombination
 Understanding this result involves exploring
genetic recombination, the production of
offspring with combinations of traits differing from
either parent
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Recombination of Unlinked Genes: Independent
Assortment of Chromosomes
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Page 300 in Textbook—shows a 50% frequency of
recombination
Gametes from yellow-round
dihybrid parent (YyRr)
 Mendel observed that combinations of traits in
some offspring differ from either parent
 Offspring with a phenotype matching one of the
parental phenotypes are called parental types
 Offspring with non-parental phenotypes (new
combinations of traits) are called recombinant
types, or recombinants
Gametes from
testcross
homozygous
recessive
parent (yyrr)
Recombination of Linked Genes: Crossing Over
yr
Yr
yR
YyRr
yyrr
Yyrr
yyRr
yr
Parentaltype
offspring
 A 50% frequency of recombination is observed for
any two genes on different chromosomes
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YR
Recombinant
offspring
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Animation: 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
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© 2014 Pearson Education, Inc.
5
Sperm
P generation
gametes
D
C
B
A
c
b
a
d
E
New Combinations of Alleles: Variation for
Natural Selection
Egg
e
f
F
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
d
E
F
Each chromosome has
hundreds or thousands
of genes. Four (A, B, C,
F ) are shown on this
one.
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.
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 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
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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
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
 Aneuploidy results from the fertilization of gametes in
which nondisjunction occurred
 Offspring with this condition have an abnormal number of
a particular chromosome
 A monosomic zygote has only one copy of a particular
chromosome
 A trisomic zygote has three copies of a particular
chromosome
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© 2014 Pearson Education, Inc.
Meiosis I
Figure
15.13—
Meiotic
nondisjunction
Nondisjunction
Video: Nondisjunction in Mitosis
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
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(b) Nondisjunction of sister
chromatids in meiosis II
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6
Polyploidy
A tetraploid (4n) mammal (Kangaroo rat)
 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
Plant Examples: Bananas = 3n; Strawberries = 8n
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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
Figure 15.14—Alterations of chromosome structure
(c) Inversion
(a) Deletion
A
B C D
E
A
B C
E
F G
B C D
A D
A
B C D E
F G
H
C B
E
F G
H
(d) Translocation
F
A
G H
B C D
A duplication repeats
a segment.
B C B
E
An inversion reverses a
segment within a chromosome.
H
(b) Duplication
A
Human Disorders Due to Chromosomal
Alterations
A
H
A deletion removes a
chromosomal segment.
 Translocation moves a segment from one
chromosome to another
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F G
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
O C D
E
F G
H
A
B P
Q
R
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Down Syndrome (Trisomy 21)
 Alterations of chromosome number and structure
are associated with some serious disorders
 Down syndrome is an aneuploid condition that
results from three copies of chromosome 21
 Some types of aneuploidy appear to upset the
genetic balance less than others, resulting in
individuals surviving to birth and beyond
 It affects about one out of every 700 children born
in the United States
 These surviving individuals have a set of
symptoms, or syndrome, characteristic of the type
of aneuploidy
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 The frequency of Down syndrome increases with
the age of the mother, a correlation that has not
been explained
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7
Figure 15.15—Down syndrome
Aneuploidy of Sex Chromosomes
 Nondisjunction of sex chromosomes produces a
variety of aneuploid conditions
 Klinefelter syndrome is the result of an extra
chromosome in a male, producing XXY individuals
(or XXXY or XXXXY)
 Monosomy X, called Turner syndrome, produces
X0 females, who are sterile; it is the only known
viable monosomy in humans
 Other types of nondisjunction: XYY, XXX
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Klinefelter’s Syndrome—usually XXY
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XXXXY, Klinefelter’s Syndrome
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© 2014 Pearson Education, Inc.
Turner Syndrome—XO
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
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8
Cri du Chat Syndrome
Disorders Caused by Structurally Altered
Chromosomes, cont.
 Certain cancers, including chronic myelogenous
leukemia (CML), are caused by translocations
of chromosomes
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© 2014 Pearson Education, Inc.
Normal chromosome 9
Normal chromosome 22
Figure 15.16—
Translocation
associated with
chronic
myelogenous
leukemia (CML)
Reciprocal translocation
Translocated chromosome 9
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 (genomic imprinting), and the other
exception involves genes located outside the
nucleus (extranuclear genes found in mitochondria
and chloroplasts.)
 In both cases, the sex of the parent contributing an
allele is a factor in the pattern of inheritance
Translocated chromosome 22
(Philadelphia chromosome)
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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
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Mitochondrial Disorders in Humans
 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—weakness, intolerance
of exercise, muscle deterioration
 Leber’s hereditary optic neuropathy—sudden
blindness in young people as young as 20s and
30s—affects oxidative phosphorylation (4 different
mutations cause the disorder)
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© 2014 Pearson Education, Inc.
9
Sex-linked Genetics Problem #1
Sex-linked Genetics Problem #2
 A man with hemophilia (a recessive, sex-linked
condition) has a daughter of normal phenotype.
She marries a man who is normal for the trait.
 Red-green color blindness is caused by a sexlinked recessive allele. A color-blind man marries a
woman with normal vision whose father was colorblind.
 What is the probability that a daughter of this mating
will be a hemophiliac?
 That a son will be a hemophiliac?
 If the couple has four sons, what is the probability
that all four will be born with hemophilia?
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 What is the probability that they will have a colorblind daughter?
 What is the probability that their first son will be
color-blind?
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