THE USE OF SELECTION EXPERIMENTS FOR DETECTING QUANTITATIVE TRAIT LOCI

Gene frequency changes following selection may reveal the existence of gene effects on the trait selected. Loci for the selected quantitativ e trait (SQTL) may thus be detected. Additionally, one can estimate th e average effect (alpha) of a marker allele associated with an SQTL fr om the allele frequency change (Delta q) due to selection of given int ensity (i). In a sample of unrelated individuals, it is optimal to sel ect the upper and lower 27 % for generating dq in order to estimate al pha. For a given number of individuals genotyped, this estimator is 0. 25i(2) times more efficient than the classical estimator of alpha, bas ed on the regression of the trait on the genotype at the marker locus. The method is extended to selection criteria using information from r elatives, showing that combined selection considerably increases the e fficiency of estimation for traits of low heritability. The method has been applied to the detection of SQTL in a selection experiment in wh ich the trait selected was pig litter size averaged over the first fou r parities, with i = 3. Results for four genes are provided, one of wh ich yielded a highly significant effect. The conditions required for v alid application of the method are discussed, including selection expe riments over several generations. Additional advantages of the method can be anticipated from determining gene frequencies on pooled samples of blood or DNA.