Medical Dictionary

Lyon hypothesis

noun Ly·on hypothesis \ˈlī-ən-\

Medical Definition of LYON HYPOTHESIS

:  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

Biographical Note for LYON HYPOTHESIS

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.


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