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Susceptibility to scrapie is known to be associated with polymorphisms at the prion protein (PrP) gene, and this association is the basis of current selective programmes implemented to control scrapie in many countries. However, these programmes might have unintended consequences for other traits that might be associated with PrP genotype. The objective of this study was to investigate the relationship between PrP genotype and coat colour characteristics in two UK native sheep breeds valued for their distinctive coat colour patterns. Coat colour pattern, darkness and spotting and PrP genotype records were available for 11 674 Badgerfaced Welsh Mountain and 2338 Shetland sheep. The data were analysed with a log–linear model using maximum likelihood. Results showed a strong significant association of PrP genotype with coat colour pattern in Badgerfaced Welsh Mountain and Shetland sheep and with the presence of white spotting in Shetland sheep. Animals with the ARR/ARR genotype (the most scrapie resistant) had higher odds of having a light dorsum and a dark abdomen than the reverse pattern. The implication of these associations is that selection to increase resistance to scrapie based only on PrP genotype could result in change in morphological diversity and affect other associated traits such as fitness.
The relative susceptibility to classical scrapie is mainly determined by polymorphisms at codons 136, 154 and 171 of the prion protein (PrP) gene (Hunter et al., 1996). Five haplotypes, defined by the combination of variants at these three codons are commonly found in sheep; scrapie eradication programmes in several countries are based on changing their frequencies. In Suffolk sheep, one codon shows variation, with the R allele associated with resistance and the Q allele associated with susceptibility. Genotyping costs may be reduced by utilizing information from relatives to infer the genotypes of other relatives. The objectives of this study were to determine the effectiveness of predicting PrP genotypes using information from relatives and to investigate the association of PrP gene with lamb performance traits in Suffolk sheep.
Scottish Blackface lamb viability records at birth, and postnatal survival from 1 day to 14 days, from 15 days to 120 days and from 121 days to 180 days were used to determine influential factors and to estimate variance components of lamb survival traits. The binary trait viability at birth was analysed using a linear model whereas the postnatal survival traits were analysed as continuous traits using a Weibull model. The data consisted of about 15 000 survival records of lambs born from 1996 to 2005 on two farms in Scotland. The models included fixed factors that had significant effects and random direct and maternal additive genetic effects and maternal litter effects for viability at birth, and sire and maternal litter effects for the postnatal survival traits. The possible effect of maternal behaviour measured around lambing on lamb survival was investigated in separate analyses. Male lambs were found to be at a higher risk of mortality than females during all periods considered. The effect of type of birth and age of dam was more important during the preweaning period than at later ages. The postnatal hazard rate was not significantly affected by the behaviour score of the dams. The genetic merit of dams had more influence on viability at birth than the genetic merit of lambs themselves. Estimates of heritability for postnatal survival traits were in the range of 0.18 to 0.33 and were significantly greater than zero. These results indicate that lamb survival can be improved through farm management practices and genetic selection. Both animal and maternal genetic effects should be considered in breeding programmes for improving viability at birth.
Several studies investigating associations between prion protein (PrP) genotypes and performance traits in several sheep breeds have recently been published. Most of these studies have failed to give conclusive results due to small numbers of animals used or a potential genotyping bias as those animals selected for PrP genotyping were chosen based on their performance (e.g. De Vries et al., 2005). Polymorphisms of the PrP gene has been linked with susceptibility to scrapie with the ARR allele generally associated with resistance and the VRQ with susceptibility. This study investigates the associations with PrP genotype for a wide range of lamb performance traits in experimental flocks of Scottish Blackface in which all animals have been PrP genotyped.
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