Susceptibility to clinical scrapie is associated with polymorphisms in the prion protein (PrP) gene. The ARR allele reduces susceptibility to clinical disease caused by all known strains of the transmissible spongiform encephalopathy (TSE) agents. For the economically important German breeds of sheep the PrP allele frequencies are well known, but this paper presents representative genotyping results for 1526 sheep from two smaller milk sheep breeds and 2446 sheep from 14 mostly indigenous land sheep breeds. The ARR allele was detected in each breed but the breed-specific ARR frequencies varied between 1 and 63 per cent. In small populations with a very low ARR frequency the ARR allele could be lost by genetic drift. A simulation study was therefore made to examine the effects of different breeding schemes in populations of different sizes on attempts to select for the ARR allele in an endangered population. In breeds in which no homozygous rams are available the breeding strategy would depend on the number of heterozygous rams, and the genotyping and selection of suitable breeding ewes would reduce the time required to achieve a highly resistant population. In general, in all the breeds a selection programme to achieve 99 per cent ARR homozygous genotypes would be feasible in six to nine generations, depending on the initial allele frequencies. In small populations the inbreeding rate may increase if no specific mating plans are developed by the breeding organisations.
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