Sex linkage

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Sex linkage is the phenotypic expression of an allele that is realted to the gender of the individual and is directly tied to the sex chromosomes. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of expressing the trait.

In mammals, the female is the homogametic sex, having two X-chromosomes (XX), while the male is heterogametic, having one X and one Y chromosome (XY). Genes that are present on the X or Y chromosome are called sex linked genes.

In birds and in some insects, the homogametic sex is male. For example, a male drake (a type of duck) has two Z-chromosomes (ZZ), while the female is heterogametic (ZW).

X-linked recessive traits are expressed in all heterogametics, but only in those homogametics that are homozygous for the recessive allele. For example, an X-linked recessive allele in humans causes Haemophilia. Haemophilia is much more common in males than females because males are hemizygous - they only have one copy of the gene in question - and therefore express the trait when they inherit one mutant allele. In contrast, a female must inherit two mutant alleles, a very infrequent event since the mutant allele is rare in the population (and her father would have been sick!).

The incidence of recessive X-linked phenotypes in females is the square of that in males (squaring a proportion less than one gives an outcome closer to 0 than the original). If 1 in 20 males in a human population are green color blind, then 1 in 400 females in the population are expected to be color blind (1/20*1/20).

Sex-linked traits are maternally inherited from carrier mothers. Each son born to a carrier mother has a 50% probability of inheriting the X-chromosome carrying the mutant allele.

In classical genetics, a Reciprocal cross is performed to test if a trait is sex-linked.