Ontogeny of X-chromosome inactivation in the female germ line

Data are presented on G6PD electrophoretic patterns in fetal ovarian preparations of G6PD heterozygotes. The results indicate that in the early or mitotic period of female germ cell development, only a single X chromosome is active in each oogonium just as is the case for X inactivated somatic tissue. However, in the later or meiotic stage, reactivation of the inactive X chromosome in each oocyte occurs so that two functional X chromosomes are present in each oocyte.