QTL analysis detects genetic loci that correlate with phenotypic variation by studying recombinant inbred lines created from crosses between two parental strains, breaking up combinations of alleles at different loci and allowing each locus's contribution to be measured independently. Structured populations are necessary because traits are usually influenced by multiple loci, so inbred strains have intermediate phenotypes due to mixtures of alleles, but recombinant inbred lines separate the alleles to identify the effects of individual loci.