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J a v a d J a m s h i d i
F a s a U n i v e r s i t y o f M e d i c a l S c i e n c e s , N o v e m b e r 2 0 1 4
S e s s i o n 5Medical Genetics
Patterns of
InheritancePart 2
2
X-Linked Dominant Inheritance
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X-Linked Dominant Inheritance Specific Features
Affected females are more frequent than affected males
All daughters of a affected male, will be affected
Females are usually less severely affected than males
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X-Linked Recessive Inheritance
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X-Linked Recessive Inheritance Specific Features
The trait or disorder should affect males almost exclusively
Are transmitted through unaffected carrier females to their sons
Male-to-male transmission is not observed
Daughters of affected males are obligate carriers
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X-inactivation (lionization)
Occurs at around 15 to 16 days' gestation, when the embryo consists of approximately 5000 cells
Normally either of the two X chromosomes can be inactivated in any particular cell.
Thereafter the same X chromosome is inactivated in all daughter cells
The inactive X chromosome exists in a condensed form during interphase (bar body)
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Variable Expression in Heterozygous Females
Images from: Emery's Elements of Medical Genetics, 14th Edition, by Peter D. Turnpenny and Sian Ellard, (2012)
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Females with X-Linked Recessive Disorders
Occasionally a woman might manifest features of an X-linked recessive trait.
Homozygosity for X-Linked Recessive Disorders
Skewed X-Inactivation
Numerical X-Chromosome Abnormalities
X-Autosome Translocations
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Y-Linked Inheritance
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Y-Linked Inheritance Specific Features
Only males are affected.
An affected male transmits Y-linked traits to all of his sons but to none of his daughters.
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Multiple Alleles ( e.g. ABO blood groups)
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Sex Influence
Autosomal traits that express more frequently in one sex than in another
Gout and presenile baldness
Hemochromatosis
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Anticipation
In some autosomal dominant disorders, the onset of the disease occurs at an earlier age in the offspring or occurs with increasing severity
A result of the expansion of unstable triplet repeat sequences
Huntington disease and myotonic dystrophy
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Mosaicism
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Mitochondrial Inheritance
Each cell contains thousands of copies of mitochondrial DNA
Mitochondrial DNA has a higher rate of spontaneous mutation than nuclear DNA
Mitochondria, and therefore their DNA, are inherited almost exclusively from the mother
A number of rare disorders with unusual combinations of neurological and myopathic features
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Mitochondrial Inheritance
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Mitochondrial Heteroplasmy
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Mitochondrial Inheritance