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The inactivation of all but one of the X chromosomes in cells of female mammals to prevent an abnormal dose of X-linked genes compared with cells of male individuals, which carry a single X chromosome. Hence, in humans, one of the two X chromosomes is normally selected for inactivation and assumes a highly condensed state as a Barr body. X inactivation is controlled by a genetic locus called the X inactivation centre. This includes an element that is transcribed as a noncoding RNA, Xist, which coats the inactivated chromosome as an inactivation signal. Some genes in the ‘silenced’ chromosome remain active, particularly in the pseudoautosomal region, a region near one of the telomeres. See also sex chromosome.

Subjects: Biological Sciences — Medicine and Health.

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