Lyonization

the naturally occurring random inactivation of an X chromosome early in embryonic development, named after the geneticist Mary Lyon, so that in consequence wherever there are two or more haploid sets of X-linked genes, all but one of the genes are inactivated, apparently at random, and have no phenotypic expression

Description
Lyonization is usual but not invariable for all loci. Its randomness explains the more variable espressivity of X-linked traits in women than in men. Lyonization also occurs in men with the Klinefelter (XXY) karyotype. See gene dosage compensation Syn Lyon hypothesis, X-inactivation