Within the elite Drysdale breed of sheep, an incidence of 26.3 of chromosome polymorphism, among a total of 327 animals studied cytogenetically, is described. Sheep were found which were both heterozygous, 2n = 53, and homozygous, 2n = 52, for a Robertsonian-type translocation. Pedigree examination of one flock suggests that the translocation chromosome had been present in some sheep of the Drysdale breed for several generations and probably since its formation. The long arms of the translocation chromosome are believed to incorporate a different acrocentric chromosome from that seen in the previously described Massey I translocation. Tentatively, the translocation has been named the Massey III translocation, and the implications of this high level of chromosome polymorphism are discussed in respect to the likely effects of impaired fertility and alternatively hybrid vigor. Two sheep which were translocation heterozygotes were found with varying degrees of segmental aplasia of their genitalia.
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