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Chromosomes and
Human Genetics
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Outline
Key concepts
Gene linkage
Sex determination in human
Sex linkage
Errors in chromosome
number
Key Concepts:
Each
gene has its own position in
sequence on a chromosome
Crossing
over allows alleles in sequence
to swap places
Allele
recombination contributes to
variations in phenotypes
Changes
in chromosomes can give rise to
genetic abnormalities or disorders
Gene linkage
I.
II.
Linkage – if genes for 2 traits are located on
the same chromosome, they are linked.
(genes on the same chromosome tend to be
inherited together)
Drosophila (Fruit fly)
P: Tan body & Long wings - TTLL
Black body & short wings – ttll
F1: Tt Ll – all Tan body & Long wings
By mating F1 with each other
F2: ¾ Tan body & Long wings (T-L-)
¼ black body & short wings (ttll),Why?
Gene linkage
[Punnett Square]
♂\♀ TL
tl
TL TTLL
TtLl
tl
TtLl
ttll
some: Tan bodies & short wings (T-ll)
black bodies & long wings (ttL-)
Crossing Over and Genetic
Recombination
Homologous
Chromosomes
Crossover
Genetic
Recombination
Sex Determination in Humans
Females
Males
XX
XY
Human Embryo and Sex
Organ Development
XY
8
Embryo
Weeks old
Gene
on Y
chromosome
governs
development of
testes
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.
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
XH
XHXH
XHXh
Y
XH Y
XhY
H
♂:
geno-ratio: ½ X Y, ½ XhY
pheno-ratio: ½ normal, ½ disease
♀ : geno-ratio: ½ XHXH, ½XHXh
pheno-ratio: all normal (½ carrier)
Patterns of
X-Linked Inheritance
X-Linked
Recessive
Inheritance
Hemophilia
Color
Blindness
Duchenne
Muscular
Dystrophy
Changes in Chromosome
Structure
Changes in Chromosome Number
1. Chromosomes:
vary in nature from 2n = 2 to 2n = 1264
human 2n = 46: ♂ 22 autosomes and XY
♀ 22 autosomes and XX
Barr bodies – every “extra” X produces a Barr body in
the nucleus (a ♀ has 1 Barr body; a ♂, none.)
2. Abnormal:
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
Down
Down
Changes in the
Number of Autosomes
Syndrome
Changes in the
Number of Autosomes
Syndrome
Turner
Changes in the
Number of Chromosomes
Syndrome
Changes in the
Number of
Chromosomes
Klinfelter
Syndrome
Changes in the
Number of Chromosomes
Klinfelter
Syndrome
Changes in the
Number of
Chromosomes
Speck
Syndrome
Changes in the
Number of
Chromosomes
XXY
Triploidy
Changes in the
Number of
Chromosomes
XXX
Triploidy
Changes in the
Number of Chromosomes
Barr
Bodies of XXXXY
Abnormal
Chromosomes
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
In Conclusion
The
parental chromosome number may
be changed
Crossing
over and random segregation
adds to adaptive variation in traits
among members of a population