The X chromosome is one of the sex chromosomes, the other being the Y chromosome. A female has two X chromosomes (XX), while a male has one X chromosome and one Y chromosome (XY). This gives rise to a problem -- since the female has two X chromosomes, a gene appearing on the X chromosomes will be overexpressed (or, if you like, the X-chromosome will be underexpressed in the male, as chromosomes normally come in pairs).

To solve this problem, we (mammals, perhaps all animals, /msg me if you know) have evolved a system in which one of the female's X chromosomes, chosen at random, shrivels up. This shriveled chromosome is called the sexual chromatin (originally), or the Barr body (after M.L. Barr, who discovered it. They are also sometimes called Bertram and Barr Bodies, as a graduate student named E.G. Bertram was co-discoverer).

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