Breeding schemes in dairy cattle are based on selection indices combining breeding values for a number of traits. The same kind of experimental crosses were subsequently used to perform a whole genome scan using a microsatellite-based marker map. The development of artificial insemination and semen cryopreservation has offered the possibility to apply extreme selection pressure on the sire side through the extensive use of elite bulls. The conventional approach towards identifying quantitative trait locus (QTL) affecting milk production in cattle would be to design an experimental cross from a limited number of founder parents originating from highly divergent lines or breeds supposed to have fixed alternative alleles at the QTL to be mapped. The parent-of-origin effect observed for the segregation of the callipyge phenotype suggests a possible role of parental imprinting. Genomics will help animal breeders select for traits that were difficult to deal with using conventional breeding strategies.