Mammalian X chromosome inactivation.

TLDR: The initial step in mammalian sexual differentiation is based on the XX: XY chromosomal system, and a single X chromosome is active in the female soma so as to eliminate gross aneuploidy effects between males and females; this is the broad outline of mammalian X-chromosome regulation.

Abstract: The initial step in mammalian sexual differentiation is based on the XX: XY chromosomal system. In order to function properly, this chromosomal mechanism must be regulated to eliminate the aneuploidy effects in somatic tissues and still insure normal sexual differentiation and development. In mammalian forms, an X-chromosome regulatory mechanism has evolved to carry out these developmental functions. The two X chromosomes in the female germ line remain active through most of their ontogeny to bring about normal ovarian function; a single X chromosome is active in the female soma so as to eliminate gross aneuploidy effects between males and females; and in the male germ line the single X chromosome is inactivated or eliminated at an apparently critical stage in spermiogenesis. This is the broad outline of mammalian X-chromosome regulation. The specifics vary in different forms: random X-chromosome inactivation in most eutherian mammals, a possible nonrandom mechanism in marsupials, and a chromosomal elimination system in the creeping vole, Micron’s oregoni.