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Aneuploid spermatocyte frequency in domestic sheep heterozygous for three Robertsonian translocations

Summary. In an analysis of the chromosomes in 332 metaphase II figures from 3 triple heterozygous rams (51,xy,t₁t₂t₃) and a further 84 figures from 3 normal rams (54,xy), there were no hypermodal cells in the normal rams but 9 such cells were found in the triple heterozygotes, giving a mean aneuploid frequency of 5·42% which was similar to the levels previously found in the single heterozygotes. Forty hypomodal cells were found in the 6 rams of which a number would have lost chromosomes due to lagging at anaphase I. Individual variation in the aneuploid metaphase II frequency was observed in the triple heterozygotes.

A significant surplus of secondary spermatocytes of normal karyotype and a deficit of 25,t₁t₃ were found in the metaphase II figures from the triple heterozygotes. There was also a significant increase of normal progeny and a deficit of 52,t₁t₃ and 53,t₂ progeny from the matings of triple heterozygous rams and normal ewes. It is possible that the significantly uneven distribution of segregation products in the triple heterozygotes might have been the result of either cell selection or degeneration during spermatogenesis.