Confirmation of the presence of a major gene for fecundity in Thoka Cheviot sheep by segregation analyses

The putative Thoka gene, with large effects on fecundity, originated in Icelandic sheep. The gene was introduced to the UK in 1985 through a programme of crossbreeding and established in Cheviot sheep (Russel et al., 1997). Ewes have been retained in the flock as putative Thoka gene carriers if they have lambed in each of the first three years and had at least two sets of twins. Progeny tests on a separate population of ewes have been used on two occasions to identify rams believed to carry the gene. Despite this complex breeding programme, the actual segregation of a gene for fecundity has yet to be unambiguously demonstrated in this flock. The purpose of this study is to use complex segregation analysis to demonstrate the existence of this gene, estimate the size of its effect and frequency of the favourable allele within the population.

Genomic contributions in a cattle introgression program

Not all commercial lines and breeds contain the best alleles for genes of economic importance. Introgression strategies transfer a gene from a non-commercial source to a commercial line by incorporating the favourable allele of a gene from the donor animals whilst retaining beneficial alleles affecting commercial traits in the recipient animals. This is achieved through generations of backcrossing and the resulting heterozygotes are selected and intercrossed to produce animals homozygous for the gene of interest. Animals produced from an introgression program can be inferior to commercial animals because of the contribution from the donor genome at loci not under selection. This genetic lag experienced in an introgression program can be partly attributed to the intercross as a higher number of animals are selected primarily upon genotype thereby reducing the selection intensity for other traits (Visscher and Haley, 1999). The chromosome segment containing the desired introgressed gene (linkage drag) can be quite large, e.g., 32 cM for a 100 cM chromosome in backcross 6 (Stam and Zeven, 1981). The aim of this study is to examine the genomic contribution of each generation to animals produced after the intercross and the level of homozygosity.

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.Key words: selection, breeding strategies, dairy cattle.

Polymorphism from environmental heterogeneity: models are only robust if the heterozygote is close in fitness to the favoured homozygote in each environment

SummaryThe lack of robustness of models of the maintenance of polymorphism in a heterogeneous environment which has been pointed out by Maynard Smith & Hoekstra (1980), applies also to models based on habitat selection, on temporal variation and on density-regulated selection. Only if (partial) dominance ‘switches’ between environments such that the fitness of the heterozygote is always close to the favoured homozygote, is there reasonable robustness. This is true for all models considered. It is argued that there are good reasons for supposing that the favourable allele at a locus may show dominance, although the experimental evidence is still scanty.