THE Lyon hypothesis postulates that the mammalian female is a natural mosaic for clones of cells with either the maternally derived X chromosome (Xm) or the paternally derived one (Xp) which is randomly inactivated early in development1,2. We have presented evidence for the dominance of the inactive Xp 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.
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