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Chapter 15
Chromosomal Basis of Inheritance
Concept 15.1: Mendelian inheritance has its
physical basis in the behavior of chromosomes
• Mitosis and meiosis were first described in the
late 1800s
• The chromosome theory of inheritance states:
– Mendelian genes have specific loci (positions)
on chromosomes
– Chromosomes undergo segregation and
independent assortment
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Figure 15.2a
P Generation
Yellow-round
seeds (YYRR)
Y
Y
R R
r

y
y
r
Meiosis
Fertilization
Gametes
R Y
y
r
Green-wrinkled
seeds (yyrr)
Figure 15.2b
All F1 plants produce
yellow-round seeds (YyRr).
F1 Generation
R
y
r
R
y
r
Y
Y
LAW OF INDEPENDENT
ASSORTMENT Alleles of
genes on nonhomologous
chromosomes assort
independently during gamete
formation.
Meiosis
LAW OF SEGREGATION
The two alleles for each
gene separate during
gamete formation.
r
R
Y
y
r
R
Metaphase I
y
Y
1
1
R
r
r
R
Y
y
Anaphase I
Y
y
r
R
2
y
Y
Y
R
R
1/
4
YR
r
1/
4
yr
y
Y
Y
Y
y
r
R
2
y
Y
Gametes
r
Metaphase
II
r
r
1/
4
Yr
y
y
R
R
1/
4
yR
Figure 15.2c
LAW OF INDEPENDENT
ASSORTMENT
LAW OF SEGREGATION
F2 Generation
3 Fertilization
recombines the
R and r alleles
at random.
An F1  F1 cross-fertilization
9
:3
:3
:1
3 Fertilization results
in the 9:3:3:1
phenotypic ratio in
the F2 generation.
Morgan’s Experimental Evidence:
Scientific Inquiry
• -specific gene / specific chromosome came
from Thomas Hunt Morgan
• Morgan’s experiments with fruit flies provided
convincing evidence that chromosomes are the
location of Mendel’s heritable factors
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• Worked with fruit flies- why?
– wild type, or normal, phenotypes that were
common in the fly populations
– mutant phenotypes -alternative to the wild type
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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 the 3:1 red:white eye
ratio, but only males had white eyes
• Morgan determined that the white-eyed mutant
allele must be located on the X chromosome
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Figure 15.4b
CONCLUSION
P
Generation
X
X
w
X
Y
w
w
Eggs
F1
Generation
Sperm
w
w
w
w
w
Eggs
F2
Generation
w
w
w
Sperm
w
w
w
w
w
w
Concept 15.2: Sex-linked genes exhibit
unique patterns of inheritance
• In humans other animals, there is a chromosomal
basis of sex determination
• Only ends of Y chromosome have regions that are
homologous with regions of the X chromosome
• The SRY gene on the Y chromosome
– Sex determining Region of Y
– If absent gonads develop into ovaries
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Figure 15.5
X
Y
• Females are XX, and males are XY
• Each ovum contains an X chromosome, while
a sperm may contain either an X or a Y
chromosome
• Other animals have different methods of sex
determination
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• 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
– Fathers- x-linked to all daughters/no sons
– Mothers-x-linked to sons and daughters
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Recessive Sex-Linked Trait
Female must be homozygous
in order to express the trait
• Fewer females with sexlinked disorders
Males are hemizygous
• a condition where only one
copy of a gene is present in
a diploid organism
• More males than females
have x-linked disorders
Examples
• Color blindness, hemophilia,
Duchenne muscular
dystrophy
Sex Linkage Problem
Color Blindness - recessive
A man who is color blind is
married to a female that
carries the color blindness
allele.
What is the probability that a
son would be color blind?
Daughter?
A daughter that is a carrier?
Sex Linkage Problem
Hemophilia- recessive
A man with hemophilia marries a
woman with normal clotting
factors but is heterozygous.
What is the probability that they will
have a … daughter with
hemophilia?
What is the probability that they will
have a… son with hemophilia?
What is the probability that their
son… will have hemophilia?
X Inactivation in Female Mammals
• one of the two X chromosomes in each cell is
randomly inactivated during embryonic
development
• The inactive X condenses into a Barr body
– Reactivated in gonad cells
– Females are a “mosaic” of cells
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Figure 15.8
X chromosomes
Allele for
orange fur
Early embryo:
Two cell
populations
in adult cat:
Allele for
black fur
Cell division and
X chromosome
inactivation
Active X
Inactive X
Active X
Black fur
Orange fur
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
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How Linkage Affects Inheritance
• Morgan did other experiments with fruit flies to
see how linkage affects inheritance of two
characters
• Saw higher # of parental types than expected if
it was a dihybrid cross- determined wing/body
color inherited together
– these genes do not assort independently, and
reasoned that they were on the same
chromosome
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Figure 15.9-4
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
(wild type)
Double mutant
TESTCROSS
b b vg vg
b b vg vg
Testcross
offspring
Eggs b vg
b vg
b vg
Wild type
Black(gray-normal) vestigial
b vg
Blacknormal
Grayvestigial
b vg
Sperm
b b vg vg
b b vg vg
b b vg vg
b b vg vg
PREDICTED RATIOS
If genes are located on different chromosomes:
1
:
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
Genetic Recombination
• nonparental phenotypes produced
• the production of offspring with combinations
of traits differing from either parent
– Recombinants
– 50% recombinants expected for genes on
different chromosomes- ie independent
assortment
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Figure 15.UN02
Gametes from yellow-round
dihybrid parent (YyRr)
Gametes from greenwrinkled homozygous
recessive parent (yyrr)
YR
yr
Yr
yR
YyRr
yyrr
Yyrr
yyRr
yr
Parentaltype
offspring
Recombinant
offspring
Recombination of Linked Genes: Crossing
Over
• Morgan discovered that genes can be linked,
but the linkage was incomplete-some
recombinant phenotypes were observed
• He proposed:
– 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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Figure 15.10b
Recombinant
chromosomes
Eggs
Testcross
offspring
bvg
965
Wild type
(gray-normal)
b vg
b vg
b vg
944
Blackvestigial
206
Grayvestigial
185
Blacknormal
b vg
b vg
b vg
b vg
b vg
b vg
b vg
b vg
Parental-type offspring
Recombinant offspring
Recombination
391 recombinants  100  17%

frequency
2,300 total offspring
b vg
Sperm
Recombinant Frequency
Problem
•
•
•
•
Total offspring – 1566
Parental types – 1472
Recombinants – 94
Frequency = (# of recombinants/Total) * 100
= %
=(distance between the two alleles
aka map units)
Recombinant Frequency
Problem
• A wild type fruit fly (heterozygous for gray
body color and red eyes) was mated with a
black fruit fly with purple eyes. What is the
recombinant frequency for these genes?
• Wild type – 721
• Black purple – 751
• Gray purple – 49
• Black red - 45
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
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• 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, represents a 1%
recombination frequency
• Map units indicate relative distance and order,
not precise locations of genes
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Errors and Exceptions
Alterations of chromosome
number
• Increase or decrease in the
number of chromosomes
from the normal diploid
number
• Occurs during anaphase of
meiosis
Alterations of chromosome
structure
• Actual chromosome is
altered
Alterations of Chromosome Number
Nondisjunction
• Certain homologous chromosomes fail to
separate during meiosis
• May occur in anaphase I or anaphase II
• Results in aneuploidy
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
(b) Nondisjunction of sister
chromatids in meiosis II
• Aneuploidy results from the fertilization of
gametes in which nondisjunction occurred
– Offspring have an abnormal number of a
particular chromosome
• monosomic zygote has only one copy of a
particular chromosome
• trisomic zygote has three copies of a
particular chromosome
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• 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
– common in plants, but not animals
– more normal in appearance than aneuploids
– Ex. Downs Syndrome
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Figure 15.15
Alterations of Chromosome Structure
Deletion
• Chromosomes lose a
fragment of DNA
• Cri du chat syndrome (#5)
– A child born with this
syndrome is mentally retarded
and has a catlike cry;
individuals usually die in
infancy or early childhood
Inversion
• Fragments break off then
reattach in reverse
orientation
Alterations of Chromosome Structure
Translocation
• Fragments reattach
to nonhomologous
chromosomes
• Chronic mylegenous
leukemia (22
switches with 9)
Concept 15.5: Some inheritance patterns
are exceptions to standard Mendelian
inheritance
• There are two normal exceptions to Mendelian
genetics
– genes located in the nucleus
– 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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Mother v. Father Inheritance
For a few mammalian traits, the phenotype depends on
which parent passed along the alleles for those traits
Genomic imprinting
• Induces intrinsic changes in chromosomes
inherited from males and females
• Silencing of certain genes that are “stamped”
with an imprint during gamete production
• Prader-Willi Syndrome (paternal)
• Angelman Syndrome (maternal)
Fragile X Syndrome
• consequence of maternal genomic imprinting
Figure 15.17b
Mutant Igf2 allele
inherited from mother
Mutant Igf2 allele
inherited from father
Normal-sized mouse (wild type)
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
• 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
• defects in mitochondrial genes prevent cells
from making enough ATP
– Make protein complexes of electron transport and
ATP synthase
– diseases that affect the muscular and nervous
systems
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