X-Inactivation

In organisms that possess multiple copies of the same chromosome, like females who have two X chromosomes, a genetic compensation process must occur during early development. X chromosome inactivation, or XCI, is the process in which one of the chromosomes in a cell is randomly silenced to prevent the over-expression of the same genes. The inactivated chromosome is condensed into a compact structure, the Barr body, that is inaccessible to transcription.

Near the middle of every X chromosome is a region called the X inactivation center, which contains two complementary sequences for non-coding RNA, called XIST and TSIX, along with other regulatory sequences. XCI is initiated by activators that promote XIST transcription on the chromosome that will be inactivated. The resulting RNA molecules coat the XIST chromosome, the one they're produced from. This step is followed by a complex set of events that includes recruitment of protein partners, reshaping of the DNA, and the production of the Barr body.

TSIX is transcribed in an antisense direction to XIST from the active chromosome, and acts to repress XIST activity by complementary binding. The inactivated chromosome in the form of the Barr body is very stable during the life of the organism and is passed on during every mitotic cell division. For instance, female calico cats get their tortoise shell coat color pattern due to the fact that one of the genes coding for fur color is X-linked, and randomly silenced in cells over different regions of their body.