Gene dosage in the evolution and function of mammalian sex chromosomes.

Ohno's early suggestions about the origin of sex chromosomes and the consequences of alterations of dosage of X and Y genes have provided an important framework for understanding sex chromosome organization, function and evolution. Here we review evidence that heteromorphic sex chromosomes evolved from an autosomal pair, and that one of the consequences of X-Y differentiation is the evolution of dosage compensation by X inactivation and upregulation of the active X, which in turn, has selected for a highly conserved X chromosome.