Associations of PrP genotype with lamb production traits in four commercial breeds of British upland and crossing sheep

Selection of sheep on PrP genotype to reduce risks of classical scrapie outbreaks is now widespread in the British sheep industry. However a recurring concern from breeders is that PrP genotype may be unfavourably associated with lamb performance. In this study we report the results from our investigations into this claim using performance data from 12,673 PrP genotyped lambs in four breeds of upland and crossing sheep: Beulah (4014), Blue Faced Leicester (725), Lleyn (5208) and North Country Cheviot (Park) (2726). We also included in the analyses performance data from around 19,000 ungenotyped lambs from the same flocks. The data were supplied by 36 breeders and comprised weights at birth, eight and twenty weeks, and ultrasonic measurements for muscle and fat depth, and deep pedigrees for all animals. Animal (direct) genetic effects and up to three maternal effects were fitted in linear mixed models for each trait. Potential associations with the PrP gene were assessed by fitting either PrP genotype or number of copies of individual alleles carried, as fixed effects. Significant associations between weight traits and PrP genotype were seen only in the North Country Cheviot (Park) breed, and were attributable to the number of copies of VRQ alleles carried. Although there was evidence of significant associations between the number of copies of individual alleles carried and weight traits at different ages in the other three breeds, these associations varied from breed to breed and were not consistent in either their direction, their magnitude or the allele concerned, and none survived Bonferroni corrections for multiple testing. The Blue Faced Leicester breed showed significant associations with PrP genotype and ultrasonic muscle depth, even after Bonferroni correction, due to the number of ARQ alleles carried. In the other breeds, associations between the number of allele copies carried and ultrasonic muscle depth were generally weak and inconsistent. No significant associations were seen for fat depth. In conclusion, few significant associations were seen. Selection on PrP genotype is unlikely to have any noticeable impact on the measured traits in these breeds.