Sex linkage
From Biocrawler, the free encyclopedia.
Sex linkage is the phenotypic expression of an allelethat is dependent on the sex of the individual and is directly tied to the sex chromosomes.
In such cases there is a homogametic sex and a heterogametic sex. In mammals the homogametic sex is female (XX) and the heterogametic sex is male (XY), thus the sex linked genes are carried on the X chromosome. In birds and some insects the homogametic sex is male and has ZZ chromosomes, with females ZW, sex-linked genes are on the Z chromosome, and "male" and "female" are exchanged.
Dominant sex-linked genes are expressed in both sexes females, even if heterozygous for the allele.
Recessive sex-linked genes are expressed in all heterogametics, but only those homogametics that are homozygous for the recessive allele. An example is the sex-linked recessive: horns in sheep that appear only in males. The recessive phenotypes of such genes are more common in males than in females; to be precise, the incidence in females is the square of that in males, so if 1/20 of the male population is green-blind, 1/400 of the female population is. (AIS does not follow this rule because it interferes with reproduction.)
Sex-linked traits are inherited through chains of carrier mothers. That is, a girl, her mother, and her mother's mother all are carriers, while some of their sons have the trait.
Some sex-linked traits in humans:
- Red and green color blindness
- Hemophilia
- Androgen insensitivity syndrome
- Duchenne muscular dystrophy
- Adrenoleukodystrophy
- Barth syndrome
- G6PD deficiency
- Lesch-Nyhan syndrome
- X-linked ichthyosis
In cats:
