Inactivation of the X chromosome is a phenomenon that occurs in women. Since women have two X chromosomes only one of them remains active and functional mixed to the chromatin while the other remains condensed and inactive. In the same 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 known 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.