X-inactivation (also called
lyonization) is a process by which one of the two copies of the
X chromosome present in
female mammals is inactivated. The inactive X chromosome is silenced by its being packaged in such a way that it has a transcriptionally inactive structure called
heterochromatin. As nearly all
female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome
gene products as
males, who only possess a single copy of the X chromosome (see
dosage compensation). The choice of which X chromosome will be inactivated is random in
placental mammals such as
humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism. Unlike the random X-inactivation in placental mammals, inactivation in
marsupials applies exclusively to the paternally derived X chromosome.
