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

Identification of allelic variants with economic importance is feasibl e via molecular genetic techniques. This information can be used to in crease the frequency of favourable alleles in dairy cattle. The effect of selection on the genotype within families in the early stages of l ife is examined. Three different strategies are considered: (1) random mating of bull sires with bull dams and with cows, with embryo select ion of young bulls and all cows; (2) random mating of bull sires with bull dams, with embryo selection of young bulls only; (3) minimizing o r avoiding matings between homozygotes for the unfavourable allele, wi th embryo selection of young bulls. Selection strategies assume the us e of reproductive technologies such as embryo transfer to produce larg e family sizes for within-family selection to be practiced. All the th ree strategies increase the frequency of the favourable allele rapidly . Strategy 1 gives the fastest increase in the frequency of the favour able allele. The increase in the frequency of the favourable allele is slower under random mating (strategy 2) than under a negative assorta tive mating (strategy 3). This is a novel example of increased selecti on response with negative assortative mating.