Molecular and Quantitative Genetics: A Useful Flirtation

Excepting the exact mathematical definition of the infinitesimal model, existing quantitative and population genetic theory is consistent with the existence of segregating quantitative trait loci (QTL) of detectable size in populations long subjected to artificial selection. Evidence is accumulating in livestock, particularly in plants, that a proportion of QTL detected using reasonably stringent criteria are real and heritable contributors to genetic variation. While theory on use of QTL strongly suggests that information on QTL will be valuable in livestock and plant improvement, empirical demonstrations are few, restricted to plants and not fully convincing. More convincing, but indirect evidence of the utility of QTL information comes from the repeatability of QTL across environments, experiments and even species, as observed in plants. Unambiguous identification of a sequence polymorphism that causes a QTL is a near impossible task in livestock given current technologies. Advances in ultra high throughput sequencing technologies and proteomics may allow a more functional approach to identifying the sequence polymorphisms that create QTL in the foreseeable future. Such identification is, however, not necessary for successful utilisation of QTL in breeding programs. It is concluded that the relationship between applied quantitative and molecular genetics goes beyond mere flirtation, but, though likely long-lasting, falls short a full marriage.