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".
The ultimate goal of complex trait dissection is to identify the actual genes involved in the trait and to understand the cellular roles and functions of these genes. Thus the purpose of the Animal QTLdb is to provide resources and tools for QTL regions of data mining, to facilitate the identification of such genes.
The accuracy and precision of locating QTL depends, in part, on the density of the linkage map created. The higher the density of the map, the more precise the location of the putative QTL. When QTL can be mapped to a relatively small chromosomal region or regions other methods, such as positional cloning, can be used effectively to isolate specific genes. Unfortunately, the denser the map, the more likely that false positive QTL will be detected with linkage map based QTL methods. More precise mapping of traits are possible with newly available genome seuqnces and genome wide association analysis (GWAS).
Most, but not all, complex traits are affectedned by more than one locus. QTL often interact in complex ways and their expression can also be influenced by non-genetic factors. Because QTL are hypotheses, they are subject to reinterpretation and revision. Because the location of QTL are provisional their nomenclature is likely to be fluid and temporary