QTLs

QTL stands for quantitative trait locus. It refers to a region of the genome that contributes to variation in a quantitative trait, such as height, yield, or blood pressure. QTLs are typically identified in populations segregating for the trait and detectable through statistical associations between genetic markers and phenotypic variation. A QTL may contain one or more genes, and the observed effect often reflects the combined action of multiple causal variants, as well as interactions with the environment.

QTL mapping relies on linkage between genetic markers and trait variation in a designed cross or natural populations. Common mapping populations include F2, backcross, recombinant inbred lines, and near-isogenic lines. Statistical methods compute LOD scores or p-values to test for association, estimate the effect size, and determine the percentage of phenotypic variance explained. Additive effects describe the average contribution of alleles, while dominance and epistatic interactions capture non-additive effects.

Two main approaches are used: linkage mapping, which exploits recombination within controlled crosses, and association mapping or genome-wide association studies (GWAS), which leverage historical recombination in diverse populations to improve resolution. Once mapped, QTL intervals can be refined by fine-mapping, candidate gene analysis, or functional studies, and the trait can be influenced in breeding programs through marker-assisted selection or genomic selection.

In functional genomics, expression QTLs (eQTLs) link genetic variation to gene expression levels; other QTL types include methylation QTLs (mQTLs) and protein QTLs (pQTLs).