Figure 1. Mapping genetic loci in F2s and RIs. A hypothetical set of inbred strains differs for the GAS locus. Strain A carries the resistance allele, strain B the sensitive allele, and they also differ for two marker sequences M1 and M2 that can be detected by PCR. Mating of A with B produces first generation F1 individuals that are genetically identical. During meiosis, the F1 chromosomes recombine (X) at random locations, sometimes more than once. Mating of F1s produces F2s with various combinations of randomly recombined chromosomes. Most chromosomes have no recombination between the M1 marker and the GAS locus. Thus, most M1A homozygotes are anesthetic resistant; most M1B:M1B animals are sensitive. Note the exception at the right where a recombination has occurred between M1 and GAS. Mating of brother-sister pairs for 20 more generations produces RIs that are homozygous at over 97% of their genome because of pairing of identical chromosomes in 1/4 of the progeny in each generation. Thus, in this example where only one locus is controlling anesthetic sensitivity, each RI strain is either resistant or sensitive and homozygous for either the A or B version of the markers. For illustrative purposes, recombination (X) is shown to occur only in F1s. In actuality, recombination continues at every generation; thus, the RIs have more recombined chromosomes than F2.