Quantitative Trait Loci (QTL) are hypotheses that specific chromosomal regions contain genes that make a significant contribution to the expression of a complex trait. QTL are generally identified by comparing the linkage (degree of co-variation) of polymorphic molecular markers and phenotypic trait measurements. The methods to localize a QTL includes whole genome scan of linkage between genetic markers and phenotypes with specific family structure designed for such analysis, and assocition analysis of markers (e.g. SNPs) with certain traits (e.g. GWAS). Therefore as a matter of fact they are genomic mappings of traits. On this database site, they are interchangeably called "QTL", "QTL/(SNP)associations".