Lyon hypothesis

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.

Medical dictionary. 2011.

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  • Lyon hypothesis — Lyon hypothesis. = inactive X hypothesis (см.). (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • Lyon hypothesis — Hypothesis, first advanced by Lyon, concerning the random inactivation of one of the two X chromosomes of the cells of female mammals. In consequence females are chimaeric for the products of the X chromosomes, a situation that has been exploited …   Dictionary of molecular biology

  • Lyon hypothesis — 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… …   The new mediacal dictionary

  • Lyon (disambiguation) — Lyon may refer to * Lyon, France * The Lyon based football club, Olympique Lyonnais * Lyon, Mississippi, USA * Lyon, Perth and Kinross, Scotland * Several counties in the United States named Lyon County * Several townships in the United States… …   Wikipedia

  • Lyon’s hypothesis — Lyon’s hypothesis. См. гипотеза Лайон. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • hypothesis — A conjecture advanced for heuristic purposes, cast in a form that is amenable to confirmation or refutation by the conductance of definable experiments and the critical assembly of empiric data; not to be confused with assumption, postulation, or …   Medical dictionary

  • Lyon — B. B. Vincent, U.S. physician, 1880–1953. See Meltzer L. test. Mary F., English cytogeneticist, *1925. See L. hypothesis, lyonization …   Medical dictionary

  • Mary F. Lyon — This article is about the 20th century British geneticist. For the 19th century American educator, see Mary Lyon. Mary Frances Lyon, FRS (born 15 May 1925 in Norwich, England) is an English geneticist, who is best known for her discovery of X… …   Wikipedia

  • World City Hypothesis — Die 1986 erstmals veröffentlichte Weltstadthypothese (orig. World City Hypothesis) von John Friedmann gilt als Begründung der modernen Weltstadt Forschung, die von einer Einteilung der Weltstädte in verschiedene Bedeutungsklassen ausgeht. Diese… …   Deutsch Wikipedia

  • Barr body — Nucleus of a female amniotic fluid cell. Top: Both X chromosome territories are detected by FISH. Shown is a single optical section made with a confocal microscope. Bottom: Same nucleus stained with DAPI and recorded with a CCD camera. The Barr… …   Wikipedia

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