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The normal phenomenon that wherever there are two or more haploid sets of X-linked genes in each cell all but one of the genes are inactivated apparently at random and have no phenotypic expression. L. is usual but not invariable for all loci. Its randomness explains the more variable espressivity of X-linked traits in women than in men. L. occurs in men with the Klinefelter (XXY) karyotype. SEE ALSO: gene dosage compensation. SYN: Lyon hypothesis, X-inactivation. [M. Lyon]
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ly·on·iza·tion (li″on-ĭ-zaґshən) [M.F. Lyon] the process by which or condition in which all X chromosomes of the somatic cells in excess of one are inactivated on a random basis. Called also X-chromosome inactivation. See also Lyon hypothesis, under hypothesis.Medical dictionary. 2011.