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More Genetics
The Chromosomal Basis of
Inheritance
A cool frog
Outline
Key concepts
Sex determination
Sex linkage
Errors in chromosomes
Key Terms
Conclusions
1
Key Concepts:
Each
gene has its own position in sequence
on a chromosome
Females
have XX and Males have XY
The
X chromosome has some genes that Y
chromosome does not have.
Changes
in chromosomes can give rise to
genetic abnormalities or disorders
Sex Determination in Humans
Females XX
Males XY
Sex Determination
1. X-0 system (some insects
including roaches and
grasshoppers)
Males no sex
chromosomes
2. Z-W system (birds, some
fishes and insects)
Sex determined by
eggs
3. Haploid-diploid system
(bees and ants)
Females from fertilized eggs
Males no father
2
Human Embryo and Sex
Organ Development
XY Embryo
– 8 Weeks old
– Gene SRY on Y
chromosome governs
development of testes
– Gene SRY codes for a
protein that regulates
many other genes
Early Human Embryo
Duct system in
early human
embryo
– Develop into male
or female
reproductive
organs
Sex linkage
The X chromosome has some genes that Y
chromosome does not have. Only one
recessive gene can get expressed. A son
receives his X chromosome from his mother
and can pass it only to his daughters. Thus,
sex-linked diseases often have a unique
pattern – skip generations.
For example: red-green color blind & hemophilia
Hemophilia is a disease in which the
blood does not clot normally. The disease is
recessively inherited and the gene is carried
on the X chromosome.
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Sex linkage
Suppose that a normal man marries a woman
who is a carrier of the disease gene of
hemophilia, what are the genotypic and
phenotypic ratios of their children?
XH = normal gene, Xh = hemophilia gene:
P: XHY X XHXh
F1:
♂\♀
XH
Xh
H
H
H
H
X
X X
X Xh
Y
XHY
Xh Y
♂:
geno-ratio: ½ XHY, ½ XhY
pheno-ratio: ½ normal, ½ disease
♀ : geno-ratio: ½ XHXH, ½XHXh
pheno-ratio: ½ normal, ½ carrier
Pedigree
analysis
Patterns of
X-Linked Inheritance
X-Linked Recessive
Inheritance
– Hemophilia A
– Color Blindness
– Duchenne Muscular
Dystrophy
4
•Y-linked inheritance
Gene located on Y chromosome
studied by Indian geneticist Dr. K. Dronamraju
Common among the populations of India
but rare in other populations
Hairy ear rims,
Proposed to be
caused by an allele
of a Y- linked gene
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Changes in Chromosome
Structure
Changes in Chromosome
Number
1. Chromosomes:
vary in nature from 2n = 2 to 2n = 1264
human 2n = 46: ♂ 22 II autosomes and XY
♀ 22 II autosomes and XX
Barr bodies – every “extra” X produces a Barr body in
the nucleus (a ♀ has 1 Barr body; a ♂, none.)
2. Abnormal: (aneuploidy and polyploidy)
Klinefelter syndrome –22II + XXY, 2n = 47(♂), 1/1000
Turner syndrome – 22II + X, 2n = 45(♀), 1/5000
Speck syndrome – 22II + XYY, 2n = 47(♂), 1/1000
Trisomy: Down syndrome – 3 #21, 2n = 47(♂), 2n =
47(♀), 1/750
Trisomy X – 22II + XXX, 2n = 47(♀), 1/1000
Changes in the
Number of Autosomes
Down Syndrome
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Changes in the
Number of Autosomes
Down Syndrome
Changes in the
Number of Chromosomes
Turner Syndrome
Changes in
the
Number of
Chromosomes
Klinfelter Syndrome
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Changes in the
Number of Chromosomes
Klinfelter Syndrome
Changes in the
Number of
Chromosomes
Speck Syndrome
Changes in the
Number of
Chromosomes
XXY Triploidy
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Changes in the
Number of
Chromosomes
XXX Triploidy
Changes in the
Number of Chromosomes
Barr Bodies of XXXXY
Carriers
Sex-linkage
Linked genes
Hemophilia
Aneuploidy
Barr body
Trisomy
Polyploidy
Down syndrome
Klinfelter syndrome
Turner syndrome
Speck syndrome
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In Conclusion
Genes
are arranged in sequence on a chromosome
Humans
have 23 pairs of homologous
chromosomes
Females
have two X chromosomes whereby males
have an X and a Y
All
other chromosomes are autosomes
The
parental chromosome number may be
changed
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