X染色體
In female organisms, a sperm containing an X chromosome fertilizes the egg, giving the embryo two copies of the X chromosome. Females, however, do not initially require both copies of the X chromosome as it would only double the amount of protein products transcribed as shown by the hypothesis of dosage compensation. The paternal X chromosome is quickly inactivated during the first few divisions. This inactive X chromosome (Xi) is packed into an incredibly tight form of chromatin called heterochromatin. This packing occurs due to the methylation of the different lysine residues that help form different histones. In humans X inactivation is a random process, that is mediated by the non-coding RNA XIST.