Starting with the F1 animals, researchers can then use different breeding schemes for subsequent analyses. One approach is the F2 design. For this approach, the F1 offspring are intercrossed—that is, brothers and sisters are mated—to generate an F2 population. In the F2 population, the alleles at the QTLs are said to be segregating, meaning that each F2 animal has a different combination of alleles at the loci contributing to the trait (see figure 4). This variability arises from the distribution of genetic material from parents to offspring as well as from a process called recombination that occurs during the specialized type of cell division that occurs when the egg and sperm are formed. Recombination is the exchange of genetic material between the two members of a chromosome pair. As a result, each individual of the F2 generation has a unique combination of the alleles found in the two progenitor strains. This diversity is reflected in the wide variation in the trait or behavior that is observed in the F2 offspring. Therefore, the F2 sample is ideal for performing analyses to determine the location of QTLs on the chromosomes (i.e., QTL mapping studies).
