Once QTL regions have been identified, most studies have proceeded to utilize planned breeding combined with analyses of the genotypes of relevant DNA markers in successive generations to develop congenic lines. These are lines of animals in which a DNA region, such as a QTL, is transferred into the genome of another strain through a series of planned matings. One can generate several related congenic lines that carry different or overlapping parts of the original QTL, which allows researchers to substantially narrow down the DNA region that contains the relevant gene(s) (see figure 5).
