IPRs - Chap 11

Although reproductive performance is one of the most critical components in swine production, it has been one of the most difficult areas for geneticists and animal breeders to improve. Traditional quantitative methods such as selection, inbreeding and cross-breeding have had limited success in this area (Rothschild, 1996). This is primarily due to the low heritabilities, sex-limited nature and insufficient phenotypic measurements for reproductive traits. Consequently, considerable variation exists for traits such as litter size, which currently ranges from approximately two to 40 pigs per litter, with average litter size ranging from nine to 16 depending on breed (Rothschild, 1996). Therefore, litter size is very well suited for approaches based on marker-assisted selection. Early investigations into the genetic basis of reproductive traits centred on the identification of blood groups and protein polymorphisms and their association with litter size. The estimation of potential pleiotropic effects of known major genes was also investigated. First evidence of such associations were obtained by Jensen et al. (1968) and Rasmusen and Hagen (1973), who reported an association between the H locus on pig chromosome 6 and litter size with an unfavourable effect of the Ha allele. Kristjansson (1964) and Imlah (1970) reported an apparent effect of alleles at the transferrin locus (pig chromosome 13) on pig fertility and prolificacy. Unfortunately, the transferrin locus effect was not confirmed by other investigators (e.g. Jensen et al., 1968; Huang and Rasmusen, 1982). Several other associations between blood group loci and reproductive traits have often led to contradictory results (e.g. 11