Fig. 23.2.

Breeding scheme to eliminate all genetic background differences except in the flanking region. Congenic and co-isogenic strains are generated either by crossing germline chimera to C57 or 129 mates, respectively. Congenic strain can be generated through multiple backcrosses or by speed congenics procedures. Crossing the coisogenic and congenic strains will yield animals of genetic backgrounds similar to that of F1 hybrids except for the region flanking the modified gene. Hence, experimental, control, and heterozygous animals can be compared with identical (except for flanking region) genetic backgrounds. Flanking allele effects can be tested by phenotypic comparison of the wild-type and heterozygous animal with the modified gene on the C57 congenic chromosome.