Investigators have used a variety of breeding schemes to identify QTLs. For most of these schemes, the first step is to mate animals from the two progenitor lines. The progeny are called F1 off-spring. If the progenitors have differing alleles at the relevant QTLs for the trait of interest, then the F1 progeny must carry one copy of each allele at all these QTLs (i.e., they are heterozygous for these QTLs). Moreover, all of the F1 animals are identical, both in their genetic makeup (i.e., their genotype) and in their observable behaviors or traits (i.e., their phenotype).
