Figure 2.

Possible mechanisms underlying male–female genetic sexual dimorphism in eutherian mammals.

The organism-wide expression of an individual gene allele is represented by block colour, with XY males in the left-hand column and XX females in the right-hand column. (A) A single allele of an X-linked gene is expressed in all cells in the male, whereas due to X chromosome inactivation (XCI), the same allele is only expressed in 50% of cells in the female. (B) XCI skewing can result in a change to the percentage of cells expressing any given X allele in females. (C) As both alleles of XCI escapee genes are expressed in females, the relative expression is increased compared to males. (D) Imprinting resulting in Xp allelic expression would be absent in males due to the absence of Xp, and would be present in 50% of cells in females. Imprinting resulting in Xm allelic expression would be present in all cells in males and 50% of cells in females. (E) Ubiquitously expressed Y-linked genes are only present in males. Abbreviations: Xm, maternally derived X chromosome; Xp, paternally derived X chromosome. Gene expression is depicted in arbitrary units, taking 1 as normal expression for a single chromosome.