Prion protein (PrP) gene polymorphisms and breeding for resistance to scrapie in Polish Merino sheep

Genetic susceptibility or resistance of sheep to scrapie is associated with polymorphisms particularly in the three codons 136, 154 and 171 of the ovine prion protein (PrP) gene. The aim of the study was to reveal the PrP haplotype and genotype distribution in the Polish Merino breed and to make a forecast of the application of a scrapie resistance breeding programme for this breed. PCR-RFLP analyses were done in order to determine PrP haplotypes based on the polymorphisms in codons 136, 154 and 171. The analysis of 98 Polish Merino sheep revealed dimorphisms only at the two codons 136 (A and V) and 171 (R and Q). 35.7% of the individuals harboured the genotype ARQ/ARQ, and 54.1% carried the genotype ARR/ARQ. The frequency of the ARR/ARR genotype was low (7.1%). A simulation study was done in order to calculate assumed frequencies of the PrP genotypes in the next generations. For the first generation the observed allele frequencies, after excluding ARQ/ARQ rams, were used in the study. For further breeding only ARR/ARR rams were used and PrP genotypes of female progeny were not considered. Employing this strategy in the Polish Merino it is possible to increase the ARR/ARR genotype frequency up to 87% in F4.