Editing X-inactivation (section)

{{short description|Inactivation of copies of X chromosome}}
{{Redirect|XCI|the Roman numerals|91 (number)}}
{{Use dmy dates|date=February 2024}}
[[Image:6-year old tortoise shell cat.jpg|thumb|right|The coloration of [[Tortoiseshell cat|tortoiseshell]] and [[calico cat]]s 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.]]
[[File:Human X-Inactivation.svg|thumb|The process and possible outcomes of random X-[[chromosome]] inactivation in female human embryonic cells undergoing [[mitosis]]. <br>
1.Early stage embryonic cell of a female human <br>
2.Maternal X chromosome <br>
3.Paternal X chromosome <br>
4.Mitosis and random X-chromosome inactivation event <br>
5.Paternal chromosome is randomly inactivated in one daughter cell, maternal chromosome is inactivated in the other <br>
6.Paternal chromosome is randomly inactivated in both daughter cells <br>
7.Maternal chromosome is randomly inactivated in both daughter cells <br>
8.Three possible random combination outcomes]]
[[Image:Sd4hi-unten-crop.jpg|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).]]
[[Image:BarrBodyBMC Biology2-21-Fig1clip293px.jpg|thumb|An interphase female human fibroblast cell.<ref>{{cite journal | vauthors = Gartler SM, Varadarajan KR, Luo P, Canfield TK, Traynor J, Francke U, Hansen RS | title = Normal histone modifications on the inactive X chromosome in ICF and Rett syndrome cells: implications for methyl-CpG binding proteins | journal = BMC Biology | volume = 2 | pages = 21 | date = September 2004 | pmid = 15377381 | pmc = 521681 | doi = 10.1186/1741-7007-2-21 | doi-access = free }}</ref> Arrows point to sex chromatin on DNA ([[DAPI]]) in cell nucleus(left), and to the corresponding X chromatin (right).<br />Left: DNA (DAPI)-stained nucleus. Arrow indicates the location of Barr body(Xi). Right: DNA associated [[histone]]s protein detected]]
[[Image:XistRNADNAFISH.jpg|thumb|right|The figure shows [[confocal microscopy]] images from a combined RNA-DNA [[Fluorescence in situ hybridization|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:.<ref name="pmid21047393"/>]]

'''X-inactivation''' (also called '''Lyonization''', after English geneticist [[Mary F. Lyon|Mary Lyon]]) is a process by which one of the copies of the [[X chromosome]] is inactivated in [[theria]]n female [[mammal]]s. 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 product]]s as males, who only possess a single copy of the X chromosome (see [[dosage compensation]]).

The choice of which X chromosome will be inactivated in a particular embryonic cell is random in [[eutheria|placental mammals]] such as humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism (its cell line). The result is that the choice of inactivated X chromosome in all the cells of the organism is a random distribution, often with about half the cells having the paternal X chromosome inactivated and half with an inactivated maternal X chromosome; but commonly, X-inactivation is unevenly distributed across the cell lines within one organism ([[skewed X-inactivation]]).

Unlike the random X-inactivation in placental mammals, inactivation in [[marsupial]]s applies exclusively to the paternally-derived X chromosome.