Final answer:
X inactivation is a vital biological mechanism in females where one of two X chromosomes is inactivated through condensation into a Barr body to prevent a double genetic dose of the X chromosome's genes. This dosage compensation is random, and once an X chromosome is inactivated in a cell, all subsequent cells derived from that cell will have the same inactive X chromosome.
Step-by-step explanation:
The phenomenon where an X chromosome is inactivated in female mammals is known as X inactivation. During embryonic development, one of the two X chromosomes in each cell condenses to form a structure known as a Barr body. This is a vital process which ensures that females, who have two X chromosomes as opposed to males who have only one, do not have a double dose of the genes encoded on the X chromosome. The process of X inactivation is random; meaning that either the maternally or paternally inherited X chromosome can be inactivated in each cell. Once an X chromosome is inactivated, all cells derived from that original cell will possess the same inactive X chromosome, providing a dosage compensation mechanism.
This biological mechanism allows for normal function despite the differences in X chromosome number between males and females. An interesting manifestation of X inactivation is seen in tortoiseshell cats, where females heterozygous for an X-linked coat color gene will express different coat colors in different areas, depending on which X chromosome was inactivated in the cell lineage that gave rise to that region of the coat.