X inactivation is the process by which all females silence one copy of their two X chromosomes. Without X inactivation, females would produce too many of the proteins on X chromosomes – a malfunction which is lethal in early development.
In mammals, two copies of the X chromosome (XX) results in a female, whereas one X chromosome and one smaller Y chromosome result in a male. The X chromosome is a large chromosome containing around 1000 genes, making it one of the largest in human and mice genomes.
The fact that females have twice the number of copies of genes on the X chromosome in comparison to males could have serious consequences for the level of expression of these genes, and resulting protein concentrations in the cell. In order to counter this, a process called X chromosome inactivation occurs.
In X chromosome inactivation, a non-coding RNA called XIST wraps around the X chromosome to be silenced. This means that one copy of the X chromosome is marked to be epigenetically silenced. X chromosome inactivation is so important that failure to carry it out is lethal in early development.
This image was inspired by research by Edith Heard.