In the early XX female zygote, both X chromosomes are active, but around the time embryonic development begins, a choice is made randomly in each cell to inactivate either the paternal or maternal X. the choice that a cell makes is preserved in all its descendants. An adult XX female has clonal populations of cells with the paternal or maternal X chromosome inactivated. The inactive X is reactivated in oocytes some time before meiosis. During spermatogenesis both the X and Y chromosomes are transiently inactivated. Adapted from Migeon (1994) Trends Genet. 10, pp. 230-235.