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Chapter 15
The Chromosomal Basis of Inheritance
Chromosomal genetics
•  Part I- Sex linkage
•  Part II: Linkage
•  Part III: Chromosomal aberrations
Dihybrid Pea Cross
seed color:
yellow (dominant)
green (recessive)
seed shape:
round (dominant)
wrinkled (recessive)
1
Yellow-round
seeds (YYRR)
P Generation
Y
Mendelian genes
have specific loci
(positions) on
chromosomes
y
Green-wrinkled
seeds ( yyrr)
y
r
Meiosis
The chromosome
theory of inheritance
states that:
r
×
R
R
Y
Fertilization
y
R Y
Gametes
r
All F1 plants produce
yellow-round seeds (YyRr)
F1 Generation
R
R
y
r
y
r
Y
Y
LAW OF SEGREGATION
The two alleles for each gene
separate during gamete
formation.
LAW OF INDEPENDENT
ASSORTMENT
Alleles of genes
on nonhomologous
chromosomes assort
independently during gamete
formation.
Meiosis
r
R
r
R
Y
y
Metaphase I
y
Y
1
It is the
chromosomes
that undergo
segregation
and
independent
assortment
1
R
r
Y
y
r
R
Y
y
Anaphase I
R
r
Y
y
Metaphase II
R
r
2
2
Y
Y
Gametes
R
1/
4 YR
F2 Generation
3
R
y
y
r
Y
Y
r
r
1/
4
y
Y
r
1/
4 Yr
yr
An F1 × F1 cross-fertilization
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:3
:3
y
R
y
R
1/
4 yR
3
:1
Linked genes
Each chromosome has 100-1000 s of genes (except Y chromosome)
Genes located on the same chromosome
that tend to be inherited together are called
linked genes
Morgan did 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
(genes are not on sex chromosome)
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EXPERIMENT
P Generation (homozygous)
Wild type
type
Wild
(gray body,
body,
(gray
normal wings
wings)
normal
b++
b++ vg++
Double mutant
mutant
Double
(black body,
body,
(black
vestigial wings)
wings)
vestigial
××
b b vg vg
vg++
F11 dihybrid
(wild type)
Double mutant
mutant
Double
TESTCROSS
TESTCROSS
××
b++ b vg++ vg
Testcross
offspring
b b vg vg
Eggs
b+ vg+
Wild type
(gray-normal)
b vg
b+ vg
b vg+
Blackvestigial
Grayvestigial
Blacknormal
b vg
Sperm
b+ b vg+ vg
b b vg+vg
b+ b vgvg
b b vgvg
PREDICTED RATIOS
If genes are located on different chromosomes:
1
:
1
:
1
:
If genes are located on the same chromosome and
parental alleles are always inherited together:
1
:
1
:
0
:
0
965
:
944
:
206
:
185
RESULTS
• 
1
From the results, Morgan reasoned that body color and wing size
are usually inherited together in specific combinations (parental phenotypes)
because the genes are on the same chromosome
However, nonparental phenotypes were also produced
Understanding this result involves exploring
genetic recombination,
production of offspring with combinations of traits differing from either parent
Recombination of Unlinked Genes:
Independent Assortment of Chromosomes
• 
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
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
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Recombination of Linked Genes:
Crossing Over
• 
Morgan discovered that
genes can be linked, but
the linkage was incomplete,
as evident from
recombinant phenotypes
• 
Morgan proposed that
some process must
sometimes break the
physical connection
between genes on the
same chromosome
• 
That mechanism was
the crossing over of
homologous
chromosomes
• 
linkage mapis a genetic map of a chromosome (an ordered list of genetic loci)
based on recombination frequencies
• 
Distances between genes can be expressed as map units; one map unit,
or centimorgan, represents a 1% recombination frequency
• 
the farther apart 2 genes are, the higher the probability that a crossover
will occur between them, and the higher the recombination frequency
body-color
cinnabar
wing-size
§  Genes that are far apart on the same chromosome can have a
recombination frequency near 50%
ü  Such genes are physically linked, but genetically unlinked,
and behave as if found on different chromosomes
• 
Recombination frequencies can
be used to make linkage maps of
fruit fly genes
• 
portray the order of genes
along a chromosome, but
does not accurately portray the
precise location of those genes
chromosome II
Using methods like chromosomal
banding, geneticists can develop
cytogenetic maps,
indicate the positions of genes
with respect to chromosomal features
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Alterations of chromosome #
•  Large-scale chromosomal alterations often lead to spontaneous
abortions (miscarriages) or cause a variety of developmental disorders
• 
nondisjunctionpairs 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
all gametes will be abnormal
½ gametes will be abnormal/ normal
•  aneuploidyresult of fertilization of gametes in which nondisjunction occurred,
offspring have an abnormal # of a particular chromosome
– 
trisomic zygote-
– 
monosomic zygote-
three copies of a particular chromosome
has only one copy of a particular chromosome
•  polyploidyan organism has more than 2 complete sets of chromosomes
tetraploid mammalfour chromosomal sets (4n)
Breakage of a chromosome
can lead to 4 types of changes
in chromosome structure:
A B C
D E
F G
H
A deletion removes a chromosomal segment.
(a) Deletion
(b) Duplication
A B C
E
F G H
A B C
D E
F G
H
A duplication repeats a segment.
A B C
B C
A B C
D E
D E
F G H
F G H
An inversion reverses a segment within a
chromosome.
(c) Inversion
A D C
B E
F G H
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.
(d) Translocation
M N O
C D E
F G H
A
B P Q
R
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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
Down Syndrome
•  Down syndrome (trisomy 21) is a condition that results from
three copies of chromosome 21
•  It affects about one out of every
700 children born in the US
•  The frequency of Down syndrome
increases with the age of the mother,
a correlation that has not been explained
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–  Nondisjunction of sex chromosomes produces
a variety of aneuploid conditions
•  Klinefelter syndromeis the result of an extra chromosome in a male,
producing XXY individuals
•  Monosomy Xcalled Turner syndrome,
produces X0 females, who are sterile;
it is the only known viable monosomy in humans
Disorders Caused by Structurally Altered Chromosomes
•  One syndrome, cri du chat ( cry of the cat ),
results from a specific deletion in chromosome 5
–  A child born with this syndrome is mentally retarded 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
Some inheritance patterns are exceptions to the
standard chromosome theory
•  There are two normal exceptions to Mendelian
genetics that involve genes located:
1. in the nucleus
2. outside the nucleus
§  mitochondria
§  plastids (chloroplasts)
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Paternal
chromosome
For a few mammalian traits,
the phenotype depends on
which parent passed along
the alleles for those traits
Such variation in phenotype is
genomic imprinting
Normal lgf2 allele
(expressed)
insulin-like
growth factor
Maternal
chromosome
Normal lgf2 allele
(not expressed)
Wild-type mouse
(normal size)
A wild-type mouse is homozygous for the normal lgf2 allele.
Normal lgf2 allele
(expressed)
Paternal
Maternal
§  involves the silencing of
certain genes that are
stamped with an imprint
during gamete production
It appears that imprinting is the result of
the methylation (addition –CH3) of DNA
Mutant lgf2 allele
(not expressed)
Normal size mouse
Mutant lgf2 allele
(expressed)
Paternal
Maternal
Normal lgf2 allele
(not expressed)
Dwarf mouse
When a normal lgf2 allele is inherited from the father, heterozygous
mice grow to normal size. But when a mutant allele is inherited
from the father, heterozygous mice have the dwarf phenotype.
Inheritance of Organelle Genes
•  Extranuclear genes are genes found in organelles in the cytoplasm
•  The inheritance of traits controlled by extranuclear genes
depends on the maternal parent 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
• 
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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