Dosage compensation of the active X chromosome

Article By:

Disteche, Christine M. Department of Pathology, University of Washington, Seattle, Washington.

Nguyen, Di Kim. Department of Medical Genetics, University of Washington, Seattle, Washington.

Last reviewed:2009

DOI:https://doi.org/10.1036/1097-8542.YB090112

Sex chromosomes have evolved from an ordinary pair of autosomes (non-sex chromosomes) in multiple lineages, including flies, worms, and mammals. Lack of recombination between the sex chromosomes has led to loss and differentiation of genes on the Y chromosome, leaving males with a single copy of most X-linked genes. Except for a small region of homology called the pseudoautosomal region, the mammalian sex chromosomes differ significantly in their gene content: For example, the human X chromosome contains about 1300 genes, whereas the Y chromosome contains about 130 genes. This striking divergence results from evolutionary forces that progressively altered the ancestral homologous pair of proto-sex chromosomes. The Y chromosome lost many genes as a result of the suppression of recombination that was needed to avoid the production of abnormal sexual phenotypes. The Y chromosome also accumulated male-advantageous genes around the testis-determining gene. The mammalian sex chromosomes apparently diverged by a stepwise mechanism that progressively suppressed recombination by means of large Y inversions.

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