Final answer:
The inactivated X chromosome in somatic cells of mammals is called a Barr body. It is a result of X inactivation during embryonic development, which compensates for the extra X chromosome in females. This process is essential for normal development and cellular function.
Step-by-step explanation:
The inactivated X chromosome in somatic cells of mammals, like the one described, is known as a Barr body. X inactivation is a crucial process by which female mammals compensate for having two X chromosomes. This process ensures that females, much like males, end up with only one active X chromosome in each somatic cell.
Early in development, one of the X chromosomes in each cell condenses to form the Barr body, thereby becoming inactive. This inactivation is random concerning whether the maternal or paternal X chromosome is inactivated. The result is that different cells will inactivate different X chromosomes, a fact that can be observed in tortoiseshell cats, where fur color variegation reflects different inactivated X chromosomes in different cells.
If a female carries an abnormal number of X chromosomes, all but one X chromosome in each cell will inactivate, but those inactivated X chromosomes can still express a few genes. During the maturation of female ovaries, the inactivated X chromosomes must reactivate.