The X chromosome occupies an exceptional place in the mammalian genome. Together with the Y chromosome, the X chromosome differentiates the sexes. Males have one X chromosome and a Y chromosome and females have two X chromosomes. Because of this fundamental genetic difference, diseases caused by genes located on the X chromosome affect males and females differently and thus present unusual inheritance patterns. Furthermore, equal dosage of expression from genes on the X chromosome is restored between males and females by a special process called X inactivation, in which genes on one of the female X chromosomes are shut down.

Figure 1. Schematic representation of the sex chromosomes of a female and male. The active X chromosome is in orange, the inactive X chromosome in purple, and the Y chromsome in green. In females, either the maternal (mat) or paternal (pat) X chromosome is inactivated in any...