Download Chapter 24: Patterns of Chromosome Inheritance

Chapter 24: Patterns of
Chromosome Inheritance
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24-1
Viewing the Chromosomes
A karyotype is a display of chromosomes
paired according to their size, location
of the centromere, and staining
patterns.
A karyotype reveals abnormalities in
chromosome number or structure.
Humans have 23 pairs of chromosomes;
22 pairs of autosomes and one pair of
sex chromosomes.
Females are XX and males are XY.
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Normal male karyotype
Fig. 24.1
2 copies of 22 autosomal
chromosomes
2 copies of 1 sex
chromosome
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Down syndrome karyotype
Fig. 24.1
(Trisomy 21)
Total of 47 chromosomes
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Changes in Chromosome
Number
Nondisjunction occurs when:
1.) both members of a homologous pair
go into the same daughter cell or
2.) when sister chromatids fail to
separate and both daughter
chromosomes go into the same
gamete.
The result is a trisomy or a monosomy.
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Nondisjunction in meiosis I
Fig. 24.2
trisomy
monosomy
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Nondisjunction in meiosis II
Fig. 24.2
monosomy
trisomy
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The Barr body is an inactive X
chromosome and is seen whenever
more than one X chromosome is
present (i.e., XX, XXY, XXX).
Cells of females function with a single
chromosome just as those of males do.
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Down Syndrome
Down syndrome is caused by trisomy 21,
a result of nondisjunction.
Nondisjunction risk increases after age
40.
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Changes in Sex Chromosome
Number
The presence/absence of a Y chromosome
determines maleness.
An abnormal number of sex chromosomes
is the result of inheriting too many or too
few X or Y chromosomes.
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Turner syndrome: females with one X
chromosome; XO.
Klinefelter syndrome: males that have
two or more X chromosomes and a Y
chromosome.
A Poly-X female has more than two X
chromosomes and extra Barr bodies.
Jacobs syndrome males are XYY.
24-11
Changes in Chromosome
Structure
A mutation is a permanent genetic
change.
Radiation, organic chemicals, or even
viruses may cause chromosomes to
break, leading to mutations.
A change in chromosome structure is a
chromosome mutation.
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Chromosomal mutations include:
1.) deletion
2.) duplication
3.) inversion
4.) translocation
24-13
Deletions
Deletions occur when part of the chromosome
is lost.
An individual with a normal chromosome from
one parent and a chromosome with a deletion
from the other parent no longer has a pair of
alleles for each trait, and a syndrome may
result.
Fig. 24.5
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Duplications
Duplication: a chromosome segment is
repeated in the same chromosome.
Fig. 24.6
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Translocation
Translocation is exchange of
chromosomal segments between two,
nonhomologous chromosomes (Ex:
part of chromosome 2 is swapped with
part of chromosome 7).
Fig. 24.7
24-16
Inversion
Inversion involves a segment of a
chromosome being turned 180
degrees; the reverse sequence of
alleles can alter gene activity.
See Fig. 24.8
A
A
B
C
D
B
F
E
E
D
F
C
NORMAL
INVERSION
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Sex-Linked Traits
Traits controlled by genes on the X or Y
chromosomes are called sex-linked
(i.e., on sex chromosomes).
An allele on the X chromosome that is in
the region where the Y chromosome
has no alleles will express even if
recessive; it is termed X-linked.
A female would have to have two
recessive genes to express the trait; a
male would only need one.
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XX = female
XY = male
X
Y
X
XX
XY
X
XX
XY
Y Chromosome
determines sex
of offspring
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Comparison of the sex chromosomes
Male
X
Female
Y
X
X
Alternate alleles
available.
Alternate alleles
available.
No alternate
alleles available.
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X-Linked Alleles
The key for an X-linked problem shows
the allele attached to the X as in:
XB = normal vision
Xb = color blindness.
Females with the genotype XBXb are
carriers because they appear to be
normal but each son has a 50% chance
of being color blind depending on
which allele the son receives.
XbXb and XbY are both colorblind.
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Cross involving an X-linked allele
A woman is not color blind but her father was color blind.
She has a child with a man that is not color blind.
Could her children be color blind?
Woman
XB Xb
Man
XB Y
XB
Color blind
Not Color blind
Y
1
XB
XB XB
XB Y
Xb
XB Xb
Xb Y
3
24-22
X-Linked Disorders
In pedigree charts that show the inheritance
pattern for X-linked recessive disorders,
more males than females have the trait.
X-linked recessive disorders include redgreen color blindness, muscular
dystrophy, and hemophilia.
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X-linked recessive pedigree chart
(Color Blindness)
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Color Blindness
Three types of cones are in the retina
detecting red, green, or blue.
Genes for blue cones are autosomal;
those for red and green cones are on
the X chromosome.
Males are much more likely to have redgreen color blindness than females.
About 8% of Caucasian men have redgreen color blindness.
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Other X-linked Disorders
Muscular dystrophy is characterized by
the wasting of muscles.
Hemophilia blood does not clot normally.
Fragile X syndrome one of the most
common forms of mental retardation.
-CGG Repeats
24-26