What is the inactivation of the X chromosome? What is a Barr body?

Inactivation of the X chromosome is a phenomenon that happens in women. As women have two X chromosomes only one of them remains active and functional mixed to the chromatin whereas the other remains condensed and inactive.

In the similar woman in some cell lineages the functional X chromosome is the one from the father and in other cell lineages the functional chromosome is the X from the mother characterizing a condition called as mosaicism (related to the X chromosome).

Under the microscope the inactive X chromosome is seen as a granule generally in the periphery of the nucleus. This granule is called the Barr body.

Image Diversity: Barr body