Effect of the prion gene on traits in sheep breeds
Scientists from the SCRAPIEFREESHEEP project combated scrapie through the use of genetics based on the selection of the ARR allele of the PrP gene. The work also addressed possible Bovine spongiform encephalopathy (BSE) risk. The team observed two genetic sources of risk for the negative effect of ARR/ARR sheep on production or functional traits called PF traits, including resistance to disease. It was possible that the PrP gene could have a pleiotropic effect on other PF traits, involving a negative direct effect on the ARR allele. If such a pleiotropic effect existed it would be expected to be stable across different breeds and populations of sheep. This risk was assessed through the use of association analysis, which was undertaken at the breed or population level. This involved making direct comparisons between different PrP genotypes for PF traits. The PrP gene could be linked to another polymorphic gene which controlled the genetic variability of a PF trait. Most commonly the PrP and PF loci were not in linkage disequilibrium. This meant that from one family of sheep to another the ARR allele could be on the same chromosome as a beneficial or non-beneficial allele of the PF gene in question. Researchers studied the pleiotropic or direct effect of the PrP gene on the PF traits of different sheep breeds or populations as well as for synthetic lines. The traits analysed included reproduction and growth as well as traits for muscle and fat depth, milk yield and content, wool and resistance to disease. The team concluded that there was no direct or pleiotropic effect by the PrP gene on the other PF traits. Therefore no linkage effect was expected between the PrP gene and other genes controlling these important traits.