Main Entry: Ly·on hypothesis

Pronunciation: \ˈlī-ən-\

Function: noun

: a hypothesis explaining why the phenotypic effect of the X chromosome is the same in the mammalian female which has two X chromosomes as it is in the male which has only one X chromosome: one of each two somatic X chromosomes in mammalian females is selected at random and inactivated early in embryonic development

Lyon, Mary Frances (born 1925), British geneticist. Lyon first proposed in 1962 a hypothesis to explain the variegated gene expression seen in female mice that were heterozygous for sex-linked genes. She proposed that in a given somatic cell of a female mouse only the genes on one of the two X chromosomes were active. The genes on one of the X chromosomes might be active in one part of a tissue, while in another part the genes on the other might function. The determination as to which X chromosome was to be active in a particular cell line was believed to occur early in embryonic development.