19. 5 mapping qtl in populations with known pedigrees. The genes that control variation in quantitative (or complex traits) are known as qualitative trait loci. Qtl have allelic variants that typically make relatively small, quantitative contributions to the phenotype. Can visualize the contributions of the alleles at a qtl to the trait value by looking at the frequency distributions associated with each genotype at a qtl. When distributions overlap, can"t determine genotype simply by looking at an individual"s phenotype (as we can for genes that segregate in mendelian ratios) Qtl mapping has revolutionized our understanding of the inheritance of quantitative traits. If the means for the genotypic classes are very close to the overall mean, there is no qtl affecting the phenotype near that marker. If the means for the genotypic classes are quite different from the overall mean and from each other, there is a. Researchers report the of the odds, or the lod score.