Prion protein genetic polymorphisms and breeding strategies in Portuguese breeds of sheep

Polymorphisms at the prion protein locus were studied in 965 animals, representative of 13 native and 3 exotic sheep breeds in Portugal. In general, ARQ was the predominant allele in most breeds, but the desirable ARR allele was present in all breeds studied. A different pattern was found in Ile de France, where ARR was predominant and VRQ had the highest frequency among the breeds analysed. In most of the breeds, especially in those of the coarse-wool type, the major hurdle in selecting for genetic resistance to clinical scrapie was the elimination of ARQ rather than VRQ alleles. Breeding schemes aiming at the creation of ARR-homozygous selection nuclei were simulated for representative breeds, and results indicate that, with intense selection, it would take between 4 years in Ile de France and 11 years in Mondegueira to obtain homozygosity in the nucleus, and the length of time needed for fixation was not affected by the type of mating (random or assortative) used. Nevertheless, intense selection for the PrP genotype alone would have undesirable consequences in terms of inbreeding, and correlated responses in production and adaptation traits should be evaluated before such a scheme is adopted.