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Chapter 15
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Chromosomal
basis for
inheritance
Mendel Genetics
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Mendel published his work in 1866
1900 his work was rediscovered.
Parallels between Mendel’s factors &
chromosome behavior
Figure 15.1
Mendel’s Genetics
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1902 Walter Sutton
Chromosomal theory of inheritance
Genes are located on chromosomes
Located at specific loci (positions)
Behavior of chromosomes during
meiosis account for inheritance
patterns
Fig. 15-2
P Generation
Yellow-round
seeds (YYRR)
Y
Y
R
r

R
Green-wrinkled
seeds ( yyrr)
y
y
r
Meiosis
Fertilization
y
R Y
Gametes
r
All F1 plants produce
yellow-round seeds (YyRr)
F1 Generation
R
R
y
r
Y
Y
LAW OF SEGREGATION
The two alleles for each gene
separate during gamete
formation.
y
r
LAW OF INDEPENDENT
ASSORTMENT Alleles of genes
on nonhomologous
chromosomes assort
independently during gamete
formation.
Meiosis
R
r
Y
y
r
R
Y
y
Metaphase I
1
1
R
r
Y
y
r
R
Y
y
Anaphase I
R
r
Y
y
Metaphase II
r
R
Y
y
2
2
Y
Y
Gametes
R
R
1/
F2 Generation
4 YR
y
r
r
r
1/
4
Y
Y
y
r
1/
yr
4 Yr
y
y
R
R
1/
4 yR
An F1  F1 cross-fertilization
3
3
9
:3
:3
:1
Fruit fly
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Thomas Morgan studied fruit flies
Drosophila melanogaster
Proved chromosomal theory correct
Studied eye color
Red is dominant, white is recessive
Crossed a homozygous dominant
female with a homozygous recessive
male
Wild type (w+)
Mutant (w)
Fruit fly
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F1 offspring were all red eyed
F2 classic 3:1 ratio red:white
phenotypes
Showed the alleles segregate
Supported the Chromosomal theory
BUT only males were white eyed
All females were red eyed or wild
type
Fig. 15-4
EXPERIMENT
P
Generation

F1
Generation
All offspring
had red eyes
RESULTS
F2
Generation
CONCLUSION
P
Generation
w+
X
X

w+
X
Y
w
Eggs
F1
Generation
w+
Sperm
w+
w+
w
w+
Eggs
F2
Generation
w
w+
w
Sperm
w+
w+
w+
w
w
w+
Fruit fly
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Eye color gene is on the Xchromosomes
Sex-linked genes:
Genes found on the sex chromosomes
X-chromosome has more genes than
Y-chromosome
Most sex-linked genes are on the Xchromosome
Human Males
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Y chromosome is very condensed
78 genes
Male characteristics
Sperm production & fertility
Males
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SRY is a gene on the Y chromosome
Sex determining region of Y
Present gonads develop into testes
Determines development of male
secondary sex characteristics
Not present then individual develops
ovaries
Females
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X chromosome has 1000 genes
One of the 2 X chromosomes is inactivated
Soon after embryonic development
Choice is random from cell to cell
Female is heterozygous for a trait
Some cells will have one allele
Some cell have the other
Females
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Barr body:
Condensed inactive X chromosome
Stains dark
Fig. 15-8
X chromosomes
Early embryo:
Two cell
populations
in adult cat:
Active X
Allele for
orange fur
Allele for
black fur
Cell division and
X chromosome
inactivation
Active X
Inactive X
Black fur
Orange fur
Sex-linked
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Mom passes gene on the Xchromosome to the son
Males have one X-chromosome
Recessive gene is expressed
Recessive alleles on the X are present
No counter alleles on the Y
Sex-linked disorders
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Mom passes sex-linked to sons &
daughters
Dad passes only to daughters
Sex-linked disorders
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Sex-linked genetic defects
Hemophilia
1/10,000 Caucasian males
Sex-linked disorders
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Colored blindness
Red-green blindness
Mostly males
Heterozygous females can have some
defects
Sex-linked disorders
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Duchenne muscular dystrophy
Almost all cases are male
Child born healthy
Muscles become weakened
Break down of the myelin sheath in nerve
stimulating muscles
Wheelchair by 12 years old
Death by 20
Linked genes
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Genes located on same chromosome
Genes are inherited together
Do not assort independently
Differs from Mendel’s law of
independent assortment
Independent assortment
Independent assortment
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Dihybrid testcross
50% phenotypes similar to parents
Parental types
50% phenotypes not similar to parents
Recombinant types
Indicates unlinked genes
Mendel’s independent assortment
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
Recombinant
offspring
Linked genes
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Test cross fruit flies
Wild-type (dihybrid)
Gray bodies and long wings
Mutants (homozygous)
Black bodies and short wings
(vestigial)
Results not consistent with genes
being on separate chromosomes
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
b b vg vg b+ b vg vg b b vg+ vg
1
:
1
:
1
:
1
1
:
1
:
0
:
0
965
:
944
:
206
:
185
Linked genes
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More parental phenotypes
Than if on separate chromosomes
Greater than 50%
Gray body normal wings or black body
vestigial
Non-parental phenotype 17%
Gray-vestigial or black-normal wings
Indicating crossing over
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
b vg
Fig. 15-10
Gray body, normal wings
(F1 dihybrid)
Testcross
parents
Replication
of chromosomes
Meiosis I
Black body, vestigial wings
(double mutant)
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
b+ vg+
Meiosis I and II
b+ vg
b vg+
b vg
Meiosis II
Recombinant
chromosomes
Eggs
Testcross
offspring
b+ vg+
b vg
b+ vg
b vg+
965
944
206
185
Wild type
(gray-normal)
Blackvestigial
Grayvestigial
Blacknormal
b+ vg+
b vg
b+ vg
b vg+
b vg
b vg
b vg
b vg
Parental-type offspring Recombinant offspring
391 recombinants
Recombination
 100 = 17%
=
frequency
2,300 total offspring
b vg
Sperm
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Genetic recombination:
New combination of genes
2 genes that are farther apart tend
to cross over more
2 genes on the same chromosome can
show independent assortment
Due to regularly crossing over
Genetic map
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Ordered list of gene loci
Linkage map:
Genetic map based on recombination
frequencies
Distance between genes in terms of
frequency of crossing over
Higher percentage of crossing over the
further apart the genes are
Centimorgan (Thomas Hunt Morgan)
A map unit
Figure 15.11
Results
Recombination
frequencies
9%
Chromosome
9.5%
17%
b
cn
vg
Figure 15.12
Mutant phenotypes
Short
aristae
0
Maroon
eyes
Black Cinnabar Vestigial Down- Brown
wings curved eyes
eyes
body
wings
48.5
16.5
Long
Red
aristae
eyes
(appendages
on head)
Gray
body
57.5
Red
eyes
67.0 75.5
104.5
Normal Normal Red
wings wings eyes
Wild-type phenotypes
Alterations in chromosomes
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Chromosome number
Chromosome structure
Serious human disorders
Alterations in numbers
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Nondisjunction
Failure of homologues or sister chromatids
to separate properly
Aneuploidy:
Gain or a loss of chromosomes due to
nondisjunction
Abnormal number of chromosomes
Occurs about 5% of the time with humans
Nondisjunction
Fig. 15-13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n–1
n–1
n+1
n–1
n
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II
n
Monosomics
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Lost a copy of a chromosome (not a
sex chromosome)
Usually do not survive
Trisomes: gained a copy of a
chromosome
Many do not survive either
35% rate of aneuploidy (spontaneous
abortions)
Polyploidy
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More than 2 sets of chromosomes
3n or 4n
Plants
Fig. 15-14
Alterations in Structure
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1. Deletion:
Missing a section of chromosome
2. Duplication:
Extra section of chromosome
Attaches to sister or non-sister
chromatids
Alterations in Structure
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3. Inversion:
Reverse orientation of section of
chromosome
4. Translocation:
Chromosome fragment joins a
nonhomologous chromosome
Fig. 15-15
(a)
(b)
(c)
(d)
A B C D E
F G H
A B C D E
F G H
A B C D E
F G H
A B C D E
F G H
Deletion
Duplication
A B C E
F G H
A B C B C D E
Inversion
A D C B E
R
F G H
M N O C D E
Reciprocal
translocation
M N O P Q
F G H
A B P Q
R
F G H
Human disorders
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Trisomes
Babies with extra chromosomes can
survive
Chromosome 13, 15, 18, 21 and 22
These are the smallest chromosomes
Trisomy 13
Trisomy 18
Down syndrome
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Trisomy 21
1866 J. Langdon Down
1 in 750 births
Similar distribution in all racial
groups
Similar distribution in chimps and
other primates
Down Syndrome
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Mental retardation
Heart disease
Intestinal problems/surgery
Hearing problems/hearing loss
Unstable joints
Leukemia
Single crease in the palm
Down syndrome
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20 years or younger 1 in 1700
20-30 years 1 in 1400
30-35 years 1 in 750
45 1 in 16
Nondisjunction
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Higher incidence in woman’s eggs than
in the men’s sperm
Woman’s eggs are in prophase I
(meiosis) when she is born
Her eggs are as old as she is!!!
Men produce new sperm daily
Down Syndrome
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Primarily from nondisjunction
Chromosome in woman’s eggs.
Therefore age of mom is very
important
Sex chromosomes
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X chromosomes fail to separate
properly
Some eggs with 2 X chromosomes
Some eggs with no X chromosome
Produce
XXX
Appears normal
Sex chromosomes
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XXY Klinefelter syndrome (1 in 500
male births)
Is a male with some female features
Sterile
Maybe slightly slower than normal
OY does not survive, need the X
chromosome
Sex chromosomes
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XO, Turner syndrome
Female that has short statue, web
neck
Sterile
1 in 5000 births
Sex Chromosomes
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XYY
1 in 1000 births
Normal fertile males
May be taller than normal
Translocation
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Philadelphia chromosome
Reciprocal exchange of chromosome
#22 and #9 exchange portions
Shortened translocated #22
CML
Fig. 15-17
Normal chromosome 9
Normal chromosome 22
Reciprocal
translocation
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
Deletion
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Cri du chat
“Cry of the cat”
Deletion of chromosome 5
Mental retardation
Small head
Die in infancy
Genomic imprinting
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Variation in phenotype
Depends on allele is inherited from
male or female
Usually autosomes
Silencing of one allele in gamete
formation
Fig. 15-18
Paternal
chromosome
Normal Igf2 allele
is expressed
Maternal
chromosome
Normal Igf2 allele
is not expressed
Wild-type mouse
(normal size)
(a) Homozygote
Mutant Igf2 allele
inherited from mother
Normal size mouse
(wild type)
Mutant Igf2 allele
inherited from father
Dwarf mouse
(mutant)
Normal Igf2 allele
is expressed
Mutant Igf2 allele
is expressed
Mutant Igf2 allele
is not expressed
Normal Igf2 allele
is not expressed
(b) Heterozygotes
Organelle genes
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Extracellular genes
Cytoplasmic genes
Mitochondria
Chloroplasts
Plastids
Figure 15.18