Sex linkage

Sex linkage is the phenotypic expression of an allele, 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). In birds and some insects the homogametic sex is male and has ZZ chromosomes, with females ZW.

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.