[Effects of different selection strategies on breeding towards scrapie resistance in the German Grey-horned Heath population].

Scrapie resistance is related to polymorphisms of the prion protein (PrP) gene. The homozygous ARR/ARR genotype of the prion protein gene is associated with high resistance against conventional scrapie infections. But this can lead to a distinct loss of genetic variation in breeds with a small population size or a low frequency of the ARR allele. For these populations an optimal breeding scheme for breeding towards scrapie resistance and an arbitrarily chosen production trait with conservation of the genetic diversity is not known yet. A simulation programme was developed, in which the structures of local populations could be used as input parameters, and the development of the ARR allele frequency, of the inbreeding coefficient, of a production trait and the genetic drift were computed in dependence of different selection schemes for scrapie resistance. An optimal strategy for the German Grey Heath population from Lower Saxony should be found. An optimal strategy in a breeding programme for ARR homozygosis should also maintain the genetic diversity present in a breed. This could be achieved when initially ARR heterozygous sheep were bred until a certain threshold value of the ARR allele was reached. The speed and costs of the breeding progress towards fixation of the ARR allele should then be optimized with regard to the aims of the breeders and breeding organisations, whereas a faster breeding towards scrapie resistance goes along with higher genotyping costs and vice versa.