Livestock Populations Offer Unique Advantages for the Molecular Dissection of Multifactorial Traits

monogenic traits has become nearly trivial, the molecular dissection of multifactorial traits—which include the majority of medically and agronomically important phenotypes—remains a major challenge. Despite the substantial resources that have been allocated to such efforts—particularly in human—a recent survey reported only 30 successful outcomes, all organisms confounded (Glazier et al. 2002). This number has to be compared with the more than 1,600 Mendelian traits for which the causal mutation has been identified in the human only. As recently stated by Rutherford and Henikoff (2003), “The nature of quantitative-trait variation is one of the last unexplored frontiers in genetics, awaiting the future cloning and definitive identification of quantitative-trait determinants, whether they be genetic or epigenetic.” Indeed, it remains largely unknown how many mutations underlie the genetic variation for a typical quantitative trait, what the nature of these mutations is (structural or regulatory; genetic or epigenetic), what the actual distribution of effect size is, what the importance of dominance, epistatic and gene-by-environment interactions is, how genetic variation is maintained, etc.