Lyon hypothesis

Ly·on hypothesis 'lī-ən- n 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 (b 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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the hypothesis that gene dosage imbalance between males and females, because of the presence of two X chromosomes in females (XX) as opposed to only one in males (XY), is compensated for by random inactivation of one of the X chromosomes in the somatic cells of females. The inactivated X chromosome becomes the Barr body (see sex chromatin).

M. F. Lyon (1925- ), British geneticist

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in mammalian somatic cells, all X chromosomes in excess of one are inactivated (in the form of sex chromatin) on a random basis at an early stage of embryogenesis, around the time of implantation. Thus the normal human female is in effect a mosaic for heterozygous X-linked genes, since the paternal X chromosome is inactivated in some cells and the maternal one in the others, and females heterozygous for an X-linked disorder often exhibit some stigmata for the condition. See also lyonization.