WITHIN-FAMILY SELECTION AT AN OTHERWISE UNSELECTED LOCUS IN DAIRY-CATTLE

Identification of allelic variants with economic importance is feasible via molecular genetic techniques. This information can be used to increase the frequency of favourable alleles in dairy cattle. The effect of selection on the genotype within families in the early stages of life is examined. Three different strategies are considered: (1) random mating of bull sires with bull dams and with cows, with embryo selection of young bulls and all cows; (2) random mating of bull sires with bull dams, with embryo selection of young bulls only; (3) minimizing or avoiding matings between homozygotes for the unfavourable allele, with embryo selection of young bulls. Selection strategies assume the use of reproductive technologies such as embryo transfer to produce large family sizes for within-family selection to be practiced. All the three strategies increase the frequency of the favourable allele rapidly. Strategy 1 gives the fastest increase in the frequency of the favourable allele. The increase in the frequency of the favourable allele is slower under random mating (strategy 2) than under a negative assortative mating (strategy 3). This is a novel example of increased selection response with negative assortative mating.