Quantitative trait mutations in cattle, sheep and pigs: A review

Most economically important traits in farm animals, such as daily gain, muscularity, meat quality, milk production, reproduction and others, are complex multifactorial traits that are controlled by an unknown number of genes combined with environmental factors. Since these traits show a continuous distribution rather than discrete values as monogenic or qualitative traits do, they are called quantitative traits and a polymorphism that affects such a quantitative trait is called a quantitative trait mutation (QTM). During the last few decades, several QTMs for different economically important quantitative traits in farm animals, such as muscularity, meat quality and milk production, were discovered. Also, for various congenital disorders, causal polymorphisms could be found or are under investigation. However, for other quantitative traits, no such QTMs have been revealed so far. An explanation for this discrepancy lies in the extent of the effect of the different polymorphisms that underlie a certain quantitative trait since this effect can vary from very small to quite large (up to 25-30% of the total phenotypic variance of a particular trait). Although new, emerging cost-effective genome-wide single-nucleotide polymorphism (SNP) genotyping techniques have become available, only QTMs that explain a large portion of the phenotypic variation are worth unravelling, especially in the light of genome-wide selection. In this review, the research for QTMs is positioned against these contemporary techniques. Moreover, an overview is given of, past and present, research efforts in identifying QTMs in farm animals and the incorporation of these polymorphisms in modern animal breeding. © CAB International 2009 (Online ISSN 1749-8848)