The study was conducted on Polish Merino (143♀ and 39♂) and old-type Polish Merino sheep (176♀ and 61♂) in Brylewo flock (Wielkopolskie Province). The examined sheep were at the age of one year. Prion protein (PrP) genotype was determined in all animals. In both breeds four alleles (ARR, ARQ, AHQ, VRQ) were found. Highly significant effect of breed and insignificant impact of gender within breed was observed in regard to the frequency of occurrence of alleles and genotypes susceptible to classical scrapie in the sheep. Eight different PrP genotypes in Polish Merino and nine genotypes in old-type Polish Merino were identified. Very high frequency of ARR/ARR genotype in old-type Polish Merino and high frequency of ARR/ARQ genotype in Polish Merino were found, with relatively significant frequency of occurrence of the genotypes containing VRQ allele. In old-type Polish Merino, three animals (of both genders) had VRQ/VRQ genotype. Breeding work involving elimination of animals encoding valine at codon 136, and introduction of rams with ARR allele to the population increased the frequency of occurrence of ARR/ARR genotype and ARR allele in the population of old-type Polish Merino. To improve the distribution of the genotypes genetically resistant to scrapie in the flock of Polish Merino only rams with ARR/ARR genotype were left. This guarantees an increase in the frequency of occurrence of genotypes genetically resistant to scrapie in the offspring.
