198. Long-term changes in genetic mean and genic variance and underlying allele frequencies in breeding programmes

Abstract

The infinitesimal model assumes that polygenic traits are influenced by (infinitely) many quantitative trait loci with (infinitely) small effects and that selection on such traits has minimal impact on the frequency of alleles at individual loci, hence minimal impact on genetic variance. Here we evaluated trends in genetic mean and genic variance for milk yield over 25 years in 9,403 proven and genotyped bulls from the Canadian Dairy Network. The results showed significant change in genetic mean and minor change in genic variance, supporting the infinitesimal model. A detailed analysis of SNP marker allele frequencies indicated sizeable changes at individual loci. We grouped the SNP markers by their initial allele frequency and direction of change, to indicate how these groups of makers contribute to changes in mean and variance in the analysed period of 25 years and predictions for the next 75 years showing sizeable changes in mean and variance.