Quantitative Trait Loci (QTL)

Most traits of agricultural importance, such as growth rate and body composition, milk yield and composition, and egg number and quality show a continuous distribution of quantitative trait measurements in a population. Such traits are termed quantitative traits, in contrast to Mendelian traits, which typically are found in a limited number of qualitatively different forms in a population (e.g., presence or absence of horns, brown or black coat color). Genetic variation in a quantitative trait is generally attributed to allelic variation at a number of genes, in contrast to the one or two genes generally found sufficient to explain genetic variation in a Mende-lian trait. Moreover, quantitative trait expression is much affected by environmental variables, again in contrast to Mendelian traits, which are generally little affected by environment. Consequently, for quantitative traits, the relationship between genotype and phenotype is complex, and the genotype of an individual cannot be inferred from its phenotype or that of its relatives. Instead, the various genes affecting a quantitative trait are individuated by mapping them to specific chromosomal locations (loci). For this reason, the term quantitative trait loci, or QTL, was proposed for the individual mapped genetic factors affecting quantitative trait value. Mapping the QTL responsible for genetic variation in traits of agricultural importance, and using the map locations to identify the actual genes involved, is a major challenge for animal genetics. Success will provide powerful tools for understanding the physiology of trait variation, and for genetic improvement of animal stocks.