correspondence

158 nature genetics• volume 21• february 1999 that new mutations may also contribute substantially to long-term selection responses11. We propose that the presence of multiple alleles and multiple consecutive mutations may be common at major trait loci under selection for many generations in domestic animals, crops or natural populations. Our recent characterization of the KIT (ref. 12) and MC1R (ref. 13) loci, which control coat-colour variation in pigs, supports this hypothesis. In this context, the identification of a third allele at the IGF2-linked QTL (ref. 14) is notable. Our results also have important practical implications for the pig breeding industry. The practice of breeding individual males with many females may in fact favour the selection of alleles at paternally expressed QTLs.