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Encyclopedia > Lyonisation

The Lyon hypothesis, conceived by Mary F. Lyon in 1961 while she was studying the genetic consequences of radiation exposure, states that in cells with multiple X chromosomes, all but one chromosome is inactivated during mammalian embryogenesis. X-inactivation leads to clumped chromatin termed Barr bodies, which are generally considered inert. This formation of Barr bodies is called Lyonization. Mary Francis Lyon (b. ... 1961 (MCMLXI) was a common year starting on Sunday (the link is to a full 1961 calendar). ... Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. ... Radiation in physics is a process of emission of energy or particles. ... Cells in culture, stained for keratin (red) and DNA (green). ... The X chromosome is one of the two sex chromosomes in mammals (the other is the Y chromosome). ... Mammalian embryogenesis is the process of cell division and cellular differentiation which leads to the development of a mammalian embryo. ... Chromatin is the structural building block of a chromosome. ... In those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male (Lyon, 2003), rendered inactive in...

In most mammals, including humans, the X chromosome that is inactivated is determined by chance. In marsupials, however, the paternal X chromosome is always inactivated. Inactivation is achieved by methylation of the X chromosome DNA, a common way in which the cell silences particular genes. Certain animals have their coloring patterns dictated by this phenomenon. For example, female calico cats have unique patterns of fur due to certain areas where different X chromosomes are inactived. The X chromosomes that are activated express different coloring genes and lead to their mosaic fur color. Orders Didelphimorphia Paucituberculata Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia Marsupials are mammals in which the female typically has a pouch (called the marsupium, from which the name Marsupial derives) in which it rears its young through early infancy. ... In biochemistry, methylation refers to the replacement of a hydrogen atom (H) with a methyl group (CH3), regardless of the substrate. ... Cells in culture, stained for keratin (red) and DNA (green). ... Tortoiseshell is a coloring found in cats caused by a combination of specific genetic traits. ... In medicine (genetics), a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one patient, where usually one of the two is affected by a genetic disorder. ...

However, there are now reports that women still express many genes from their inactive X chromosomes, and that different women express different genes from the inactive X. Research by Carrel and Willard (2005) indicates that 15% of the genes on the inactive X chromosome are actually active across all women, and a further 10% of genes were switched on in some women. Many of these genes are dominant genes covering for defective recessive alleles on the active X chromosome to cover such sex-linked disorders as haemophilia and color blindness. An allele is any one of a number of viable DNA codings occupying a given locus (position) on a chromosome. ... Haemophilia or hemophilia is the name of any of several hereditary genetic illnesses that impair the bodys ability to control bleeding. ... Color blindness in humans is the inability to perceive differences between some or all colors that other people can distinguish. ...

See also

In those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z, a Barr body is the inactive X chromosome in a female cell, or the inactive Z in a male. ... The XY sex-determination system is a well-known sex-determination system. ... Dosage compensation is a genetic regulatory mechanism which operates to equalize the phenotypic expression of characteristics determined by genes on the X chromosome so that they are equally expressed in the human XY male and the XX female. ... Epigenetics is the study of reversible heritable changes in gene function that occur without a change in the sequence of nuclear DNA. It is also the study of the processes involved in the unfolding development of an organism. ...


  • Carrel L, Willard HF. X-inactivation profile reveals extensive variability in X-linked gene expression in females.. Nature 2005;434:400-404. PMID 15772666.
  • Lyon MF. Gene action in the X chromosome of the mouse (Mus musculus L). Nature 1961;190:372-373. PMID 13764598.



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