Quantitative Trait Locus Mapping: Some Biological and Statistical Considerations

Many important traits in plant and animal populations such as yield, quality, and resistance/tolerance to biotic and abiotic stresses are controlled by many genes with small effects and are known as quantitative traits (also ‘polygenic,’ ‘multifactorial’ or ‘complex’ traits). The regions within genomes that contain genes associated with variation of quantitative traits are known as quantitative trait loci (QTL). A key development in the field of complex trait analysis was the establishment of large collections of molecular/genetic markers, which could be used to construct detailed genetic maps of both experimental and domesticated species. These maps provided the foundation for the modern-day QUANTITATIVE TRAIT LOCUS (QTL) mapping methodologies. The identification of QTLs can help to understand how many genomic regions significantly contribute to the trait variation in a population and how much variation is due to additive, dominant or epistatic effects of QTLs. Although the basic principle of QTL mapping has been established in Sax’s work on beans, the identification of QTLs based only on conventional phenotypic evaluation is possible. A major breakthrough in the characterization of quantitative traits was initiated by the development of RFLP markers which created opportunities to select QTLs. After that generally, biparental populations are used to map QTLs, in which the marker genotype and trait phenotype data are analyzed to detect the association between the two. The advent of molecular marker technology and the development of detailed linkage maps in various organisms made it possible to dissect QTs into discrete genetic factors. This review focuses the discussion on the biological considerations and statistical methods used for mapping QTLs