An alternative approach that is also commonly used to identify QTLs, particularly using the B6 and D2 progenitors, is the analysis of recombinant inbred (RI) lines. To generate RI lines, brothers and sisters from the F2 generation are mated, and this inbreeding is continued for 20 or more generations. As a result of the inbreeding, independent RI lines are created where all animals within an RI line are genetically identical but all RI lines are genetically distinct from one another (see figure 4). Nevertheless, the RI lines are all related to each other because they were derived from the same two progenitors. The RI approach can be a particularly cost-effective means to initially localize QTLs because the genotypes for many RI lines are known and readily available. Thus, an investigator only needs to test the trait(s) of interest in the animals of a set of RI lines and then correlate the trait results with the available genotypic data to identify putative QTLs.
