QTLs are associated with:
Quantitative Trait Loci are genomic regions contributing to continuously varying traits that show distribution overlapping between phenotypes rather than discrete classes. Traits like grain yield, plant height, days to flowering, oil content, and drought tolerance are governed by many genes of small effect plus environmental modification, producing normal bell curve distribution in segregating population. Each QTL explains proportion of phenotypic variance, detected via statistical linkage between marker genotype and trait mean in mapping populations such as F2, RILs, or DH using interval mapping. Unlike Mendelian traits where single gene causes qualitative difference, QTLs exhibit additive effects, epistasis, and genotype-by-environment interaction requiring multi-environment phenotyping. Mapping exploits molecular markers across chromosome, interval mapping calculates LOD score for association. Cloning QTLs like Hd1 for heading date reveals molecular basis underlying continuous variation, bridging quantitative genetics and molecular biology frameworks and enabling map-based cloning and MAS for complex traits. Composite interval mapping and mixed linear models account for population structure and kinship, improving QTL detection power; fine mapping with near isogenic lines and map based cloning isolates causal genes underlying QTLs, such as SUB1A for submergence tolerance, demonstrating molecular deciphering of continuous variation into discrete genes for breeding.
Ref: Lynch M & Walsh B Quantitative Genetics; Tanksley SD 1993 – Mapping polygenes continuous traits – QTL Genetics