QTL (Quantitative Trait Locus)

Variation in most traits is controlled polymorphisms at a number of genetic loci together with the influence of various environmental factors. These traits are termed 'complex' or 'multifactorial' and may have phenotypic distributions that are either quantitative (e.g., height in humans or growth rate in livestock or crop plants) or discrete (e.g., diseased vs. healthy). A particular locus in the genome with two or more alleles controlling complex trait variation is termed a quantitative trait locus or QTL. Such loci can be mapped in the genome by their association with polymorphic genetic markers (such as single-nucleotide polymorphisms (SNPs) or microsatellites) by using linkage analysis in designed crosses or families or by association in a population sample (i.e., in a genome-wide association study or GWAS). The particular QTLs segregating for a trait may vary between different crosses and populations. The ability to detect segregating QTLs may be influenced by interactions with other genetic variation (epistasis) or with environmental factors. Statistical approaches are required for data analysis in order to map QTLs, and QTLs that control only a small proportion of the variance may be very difficult to detect and map accurately, requiring very large studies. The relative inaccuracy of mapping combined with our limited ability to predict the phenotypic consequences of genetic changes makes moving from QTL location to the identification of the underlying causal DNA variation very challenging and relatively rarely achieved. Nonetheless, knowledge on QTLs and their location provides understanding of genes and pathways involved in both sickness and in health, pointing to potential treatments for diseases and in plants and livestock facilitating the breeding of improved strains and lines.