Sex-linked inheritance

Inheritance related to sex chromosomes exists in three types – X-linked dominant, X-linked recessive and Dutch, i.e. Y-linked.

This is fundamentally a deviation from Mendel's laws - the monitored gene is not located on the autosome, but on the gonosome.

Inheritance gonosomally dominant

 * women are affected twice as often as men (they can inherit the disease from both parents)
 * affected person has at least one affected parent = vertical type of inheritance
 * typical diseases:
 * vitamin D resistant rickets
 * incontinentia pigmenti
 * gametes combination XY and mutated = incapable of further development → one part of males does not develop → arise:
 * 2 parts female: 1 part male
 * ← shift in the sex ratio
 * For clarity:

-         a heterozygous woman has affected sons and daughters with a 50% risk

-         the affected man has all his daughters affected and his sons are healthy (they have Y from him)

Inheritance gonosomally recessive

 * trait linked to a chromosome = practically only men are affected − women are only carriers (healthy – mostly)
 * a man is hemizygous for the gene, so he only needs one chromosome for the disease, while a woman would have to inherit the diseased allele from both parents
 * typical diseases:
 * hemophilia A, B
 * Daltonism (color blindness)
 * Duchenne muscular dystrophy - fatal under 20 years of age ( dystrophin production defect )
 * For clarity:

-         women (heterozygotes) are carriers of the disease

- a typical GR family tree shows the characteristic skipping of one generation, i.e. that the affected male has all carrier daughters (healthy heterozygotes) and all healthy sons (they get Y from the father)

-         sons of carrier women have a 50% risk of disability In the case of a combination of a carrier mother and a sick father, half of the daughters and sons are sick, half of the daughters are carriers and half of the sons are healthy.


 * a typical family tree with generation-skipping – a sick father has only healthy sons and carrier daughters, who in turn can have sick sons

Lyonization

 * changes on the X chromosomes, one of which is inactive = genes will not manifest in the phenotype
 * inactivation during embryonic development − it is random which of the X chromosomes will be inactive
 * the resulting phenotype of the heterozygote, therefore, depends to some extent on how the lyonization took place (partial manifestation of the disease in carriers)

Gonosomal inheritance Y = Holland type
More detailed information can be found on the Dutch Inheritance page .


 * the Y chromosome is acrocentric = the centromere is near the end = the smallest chromosome in the human karyotype
 * no hereditary disease transmitted through the Y chromosome has yet been found
 * area for male sex determination (on short arms) − near it lies the SRY area ← is responsible for spermatogenesis

related articles

 * Allelic interactions