An extra copy of the X chromosome, unlike autosomes, exerts only minor effects on development in mammals including man and mice, because all X chromosomes except one are genetically inactivated. Contrary to this contention, we found that an additional maternally derived X (XM) chromosome, but probably not a paternally derived one (XP), consistently contributes to early death of 41,XXY and 41,XXX embryos in mice. Because of imprinted resistance to inactivation, two doses of XM remain active in the trophectoderm, and seem to be responsible for the failure in the development of the ectoplacental cone and extraembryonic ectoderm, and hence, from early embryonic death. Discordant observations in man indicating viability of XMXMXP and XMXMY individuals suggest that imprinting on the human X chromosome is either weak, unstable or erased before the initiation of X-inactivation in progenitors of extraembryonic membranes.

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