Dosage compensation and gene expression on the mammalian X chromosome: one plus one does not always equal two

Counting chromosomes is not just simple math. Although normal males and females differ in sex chromosome content (XY vs. XX), X chromosome imbalance is tolerated because dosage compensation mechanisms have evolved to ensure functional equivalence. In mammals this is accomplished by two processes--X chromosome inactivation that silences most genes on one X chromosome in females, leading to functional X monosomy for most genes in both sexes, and X chromosome upregulation that results in increased gene expression on the single active X in males and females, equalizing dosage relative to autosomes. This review focuses on genes on the X chromosome, and how gene content, organization and expression levels can be influenced by these two processes. Special attention is given to genes that are not X inactivated, and are not necessarily fully dosage compensated. These genes that "escape" X inactivation are of medical importance as they explain phenotypes in individuals with sex chromosome aneuploidies and may impact normal traits and disorders that differ between men and women. Moreover, escape genes give insight into how X chromosome inactivation is spread and maintained on the X.