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

 For more information see Gonosomal Dominant Inheritance.

• 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)

Sick mother			½ daughter sick ½ daughters healthy ½ sons sick ½ sons health
	X A	X
X	X and X	XX
Y	X AND Y	XY

Sick father			all daughters ill all sons healthy
	X	X
X A	X and X	X and X
Y	XY	XY

Inheritance gonosomally recessive

 For more information see Gonosomal recessive inheritance.

• 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

Carrier mother			½ daughter of carrier ½ daughters healthy ½ sons sick ½ sons health
	X a	X
X	X and X	XX
Y	X and Y	XY

Sick mother			all daughters are carriers all sons sick
	X a	X a
X	X and X	X and X
Y	X and Y	X and Y

Sick father			all daughters are carriers all sons healthy
	X	X
X a	X and X	X and X
Y	XY	XY

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

Deviations from the normal pedigree

Lyonization

 For more information see 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

Links

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• Allelic interactions