X-inactivation

thumb|right|The coloration of tortoiseshell cats is a visible manifestation of X-inactivation. The black and orange [[alleles of a fur coloration gene reside on the X chromosome. For any given patch of fur, the inactivation of an X chromosome that carries one allele results in the fur color of the other, active allele.]] thumb|The process and possible outcomes of random X-chromosome inactivation in female human embryonic cells undergoing [[mitosis. 1.Early stage embryonic cell of a female human 2.Maternal X chromosome 3.Paternal X chromosome 4.Mitosis and random X-chromosome inactivation event 5.Paternal chromosome is randomly inactivated in one daughter cell, maternal chromosome is inactivated in the other 6.Paternal chromosome is randomly inactivated in both daughter cells 7.Maternal chromosome is randomly inactivated in both daughter cells 8.Three possible random combination outcomes]] thumb|Nucleus of a female cell. Top: Both X-chromosomes are detected, by Fluorescence in situ hybridization|FISH. Bottom: The same nucleus stained with a DNA stain ([[DAPI). The Barr body is indicated by the arrow, it identifies the inactive X (Xi).]] thumb|An interphase female human fibroblast cell. Arrows point to sex chromatin on DNA (DAPI) in cell nucleus(left), and to the corresponding X chromatin (right).Left: DNA (DAPI)-stained nucleus. Arrow indicates the location of Barr body(Xi). Right: DNA associated [[histones protein detected]] thumb|right|The figure shows confocal microscopy images from a combined RNA-DNA FISH experiment for [[Xist in fibroblast cells from adult female mouse, demonstrating that Xist RNA is coating only one of the X-chromosomes. RNA FISH signals from Xist RNA are shown in red color, marking the inactive X-chromosome (Xi). DNA FISH signals from Xist loci are shown in yellow color, marking both active and inactive X-chromosomes (Xa, Xi). The nucleus (DAPI-stained) is shown in blue color. The figure is adapted from:.]]

X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged into a transcriptionally inactive structure called heterochromatin. As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome (see dosage compensation).