X-inactivation

"XCI" redirects here. For the Roman numerals, see 91 (number).

The coloration of tortoiseshell and calico 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.
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
Nucleus of a female cell. Top: Both X-chromosomes are detected, by 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).
An interphase female human fibroblast cell.[1] 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
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:.[2]

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).

The choice of which X chromosome will be inactivated in a particular embryonic cell is random in 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 marsupials applies exclusively to the paternally-derived X chromosome.

  1. ^ Gartler SM, Varadarajan KR, Luo P, Canfield TK, Traynor J, Francke U, Hansen RS (September 2004). "Normal histone modifications on the inactive X chromosome in ICF and Rett syndrome cells: implications for methyl-CpG binding proteins". BMC Biology. 2: 21. doi:10.1186/1741-7007-2-21. PMC 521681. PMID 15377381.
  2. ^ Cite error: The named reference pmid21047393 was invoked but never defined (see the help page).