X Chromosome Inactivation
In mammals, including humans, females have two X chromosomes and males have a single X along with the Y chromosome. Most of the genes on the X chromosome are expressed in both males and females and are necessary for the normal development and functions of all the cells in the body. Therefore, to compensate for the difference in the number of copies of the genes on the X chromosomes between the two sexes, one of the X chromosomes in female mammals is inactivated. Thus, dosage compensation is brought about by random inactivation of one the two X chromosomes in each cell very early in development when the fertilized egg has around 200 cells. However, once the paternally or the maternally inherited X chromosome is inactivated in a cell at this stage, the same X chromosome will be inactivated in all the cells derived from this cell throughout development and in the adult. Thus, the random inactivation of the X chromosome is an...