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

The Inactivation of the X chromosome is a phenomenon that occurs in women. Ever 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 and 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 called as mosaicism (related to the X chromosome) and 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.