Females have two X chromosomes, whereas males have only one; and yet, males undergo normal development. How can this be? The answer lies in an observation that was made by the geneticist Mary Lyon. Female mammals have a dark region located just inside the nucleus, along the nuclear membrane. The object is called a Barr body. The Barr body is an inactivated X chromosome that stays condensed throughout the cell cycle. By inactivated, it is meant that most of the genes on the chromosome are not expressed and cannot be used to make proteins. Therefore, like the cells in males, females have only one functioning X chromosome and they contribute only one dose (copy) of the genes on that chromosome. Inactivation of the X chromosome in females compensates for the extra copy, thereby making the production of proteins in the cells of males and females the same.
How is the X chromosome inactivated in female cells? It is a process that happens early in development, when one of the two X chromosomes in the cells of the embryo becomes inactivated. It appears to be a random process, with about 50% of the cells inactivating the maternal X and 50% inactivating the paternal X. This animation of X-inactivation during development is from the Howard Hughes Medical Institute.