In the earliest phases of female embryogenesis, gene expres-sions from both the maternal X chromosome (donated by the egg) and the paternal X chromosome (donat-ed by the sperm) exist.

• 在内细胞团,X染色体灭活是随机的;但在胎盘组织中只有来自卵细胞的X染色体处于可转录的活性状态。