Non-random inactivation of X chromosome in the rat yolk sac

Abstract

THE Lyon hypothesis postulates that the mammalian female is a natural mosaic for clones of cells with either the maternally derived X chromosome (X^m) or the paternally derived one (X^p) which is randomly inactivated early in development^1,2. We have presented evidence for the dominance of the inactive X^p in extraembryonic regions of 7.5- and 8.5-d mouse embryos heterozygous for Cattanach's translocation³ in which the two X chromosomes could be readily identified⁴. Though we presumed that this might not be an exceptional phenomenon restricted to mice bearing this X-autosome translocation, this has been difficult to confirm because of the lack of a system suitable for experiments. Here we report further cytological evidence that the inactive X is predominantly paternal in the yolk sac of the laboratory rat.