Principle of the extreme-QTL (X-QTL) analysis. X-QTL analysis can be performed on a simple Mendelian inherited trait (for instance drug susceptibility (dominant trait) and drug resistance (recessive trait)) using pools of individuals (grey circle). Individuals with susceptible phenotype (yellow balls) and resistant phenotype (dark blue balls) are crossed to generate F1 and F2 individuals. The F2 individuals are divided in two pools: one under no selection (without drug treatment, light blue circle) and another under selection (with drug treatment, red circle). The allele frequency is equal in unselected (light blue line) and selected (red line) pools across the genome, except in the genome region (chromosome 6 here) containing the critical genes that underlie the selected trait.