Genetic improvement schemes in livestock are based on the assumption that the expression of relevant genes is independent of parent of origin. Until now no evidence has been found to reject this assumption. The present study on three purebred pig populations, however, shows that a significant proportion of the phenotypic variance in backfat thickness (5-7%) can be explained by genes subject to paternal imprinting. The implication is that there are genes affecting backfat that are expressed only when derived from the paternal gamete. Paternal imprinted effects explained 1-4% of the phenotypic variation for growth rate. Maternal imprinted effects were heavily confounded with heritable maternal environmental effects. When modelled separately, these effects explained 2-5% and 3-4% of the phenotypic variance in backfat thickness and growth rate, respectively. Gametic imprinting may have consequences for the optimization of breeding programmes, especially in crossbreeding systems with specialized sire and dam lines.
