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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 rare. Potentially expressed in both sexes, a sex preference may still exist.
X-linked recessive 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.
- Androgen insensitivity syndrome
- Barth syndrome
- Becker's muscular dystrophy
- Color blindness: Red and green
- Duchenne muscular dystrophy
- Fragile X syndrome
- G6PD deficiency
- Kennedy disease
- Lesch-Nyhan syndrome
- Ornithine transcarbamylase deficiency
- Wiskott-Aldrich syndrome
- X-linked agammaglobulinemia
- X-linked ichthyosis
- cats: Orange and black fur
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