1. Very early in the development of female mammals, both Paternal X chromosomes are active chromosome AT FERTILIZATION 2. As development proceeds, one X chromosome is inacti ale zygote with X chromosomes % vated in each cell Maternal X chromosome 3. This inactivation is random. In some cells, the paternally derived x chromosome is inactivated; in other cells, the EARLY CLEAVAGE maternally derived X chromosome is shut down Both X chromosomes active in all cells 4. This process is irreversible. Once an X chromosome has (3 been inactivated in a cell. the same X chromosome is in activated in all that cell s progeny. Since X inactivation happens relatively early in development, an entire region IMPLANTATION of cells derived from a single cell may all have the same X Random inactivation chromosome inactivated. Thus, all tissues in female of one x chromosom in all cells of the inner mammals are mosaics of two cell types ell mass, which will Barr bodies Barr body in the nucleus of a human oral epithelial cell stained with Cresyl violetBarr body in the nucleus of a human oral epithelial cell stained with Cresyl violet