Observations on the incidence of Robertsonian translocations and associated testicular changes in a flock of New Zealand Romney sheep

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Abstract
In the present paper preliminary observations are described on the incidence of the Massey I Robertsonian translocation in Romney sheep. In the established “M” pedigree flock an incidence of 4.6% of translocation heterozygotes was found in 152 animals examined randomly, compared with no such translocation in 246 sheep of varying breed from several countries. From pedigree follow-up a total of 25 translocation animals were found in an overall total of 180 animals studied. The origin of the translocation can be traced to two other popular pedigree flocks, “BH” and “MDW”, so that it could be distributed extensively in the Romney breed in New Zealand. Five rams with the translocation found in the “M” pedigree flock also had testicular abnormalities. Only three rams with the translocation from the same flock had normal testes. It is suggested that a “sterility” gene, which manifests itself by incomplete testicular development, may be associated in some cases with the translocation chromosome or its homologues and be present in either translocation heterozygotes or nontranslocation-carrying animals. Ewes heterozygous for the translocation appeared phenotypically normal. From leukocyte cultures one translocation heterozygote ewe was shown to be a normal translocation mosaic. A phenotypically normal ewe was found also from the present survey with a second type of Robertsonian translocation (Massey II), involving two of the larger acrocentric chromsomes and distinct from the Massey I translocation.

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