Genomic imprinting of the X chromosome: a novel mechanism for the evolution of sexual dimorphism.

Genomic imprinting is the differential marking of maternally and paternally inherited alleles of specific genes or chromosome regions during gametogenesis. The imprint silences the allele from 1 parent. A number of imprinted genes that are expressed in the brain have been identified in humans. They control the actions of other genes or regulate their products. Sexual dimorphism in the vertebrate brain is conventionally thought to be due to the epigenetic action of gonadal hormones. Sex differences could also reflect the actions of an imprinted X-linked locus. Until very recently no imprinted gene had been described on the X chromosome in humans. Here the implications of such a mechanism for the evolution of sexual dimorphism are discussed.