X-inactivation

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 it being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin. As female mammals have two X chromosomes, X-inactivation causes them not to have twice as many X chromosome gene products as males, which 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 mice and 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.