Preferential inactivation of the paternally derived X chromosome in the extraembryonic membranes of the mouse

Abstract

RANDOM X inactivation makes the female mammal a natural mosaic for clones of cells having either the maternally derived X (X^m) or paternally derived one (X^p) which is genetically inactive¹. There are, however, instances in which inactivation is obviously not random^2–7. Non-randomness was inferred from studies made on differentiated cells remote from early embryonic cells in which inactivation occurred. Thus it is not clear whether the randomness of the X inactivation process was influenced or whether cell selection occurring after random inactivation was responsible for the ultimate non-random appearance^4–9. In an effort to determine the embryonic stage at which the X chromosome initiates differentiation in famale mouse embryos heterozygous for Cattanach's translocaton¹⁰, we found that the mosaic composition was consistently biased in extraembryonic membranes, whereas it was not necessarily so in the embryonic body.