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A functional 'sex-reversed’ oocyte in the mouse

Two fertile female chimaeras derived by aggregation of morulae from the inbred mouse strains AKR/J and CBA/H-T6 were found to be of mixed sex chromosome constitution, XX/XY. The XY component of both chimaeras was AKR (albino). The records showed that one of the chimaeras had two litters totalling seven young including an exceptional son with an albino coat. This implied that the maternal gamete had originated from the XY component. Other possible explanations are examined and effectively excluded. The exceptional son was sterile and was later found to have the rare 41, XXY (‘Klinefelter’) karyotype. The possibility of functional reversal of germ cells in mouse chimaeras had hitherto been dismissed on the basis of the apparently undisturbed sex ratio of their progeny and the great excess of single-phenotype progenies in test matings. This evidence is re-examined and shown to be indecisive for female chimaeras.