Double X

Sex linkage is the phenotypic expression of an allele that is related to the sex of the individual and is found on 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. Since, in humans, there are many more genes on the X than there are on the Y, there are many more X-linked traits than there are Y-linked traits.

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, male ducks have two Z chromosomes (ZZ), while females are 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 less frequent event since the mutant allele is rare in the population.

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).

X-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. There are a few Y-linked traits; these are inherited from the father.

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

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