Quantitative trait loci associated with brain weight in the BXD/Ty recombinant inbred mouse strains

Adult C57BL/6J (B6) male mice had 37% heavier brains than did DBA/2J (D2) mice, while their body weights did not differ. The BXD recombinant inbred (RI) series of 20 strains, derived from a cross between B6 and D2 inbred strains, was used as the initial screen to determine significant associations between male brain weight and brain:body weight ratio, with allelic variation at 360 known marker gene loci. For brain weight, this yielded five candidate chromosome regions, each reflecting a possible quantitative trait locus (QTL) site affecting brain weight. The second step was to test as many of these five as possible using standard (non-RI) inbred strain data for brain weight previously reported in the literature. For this purpose, only strains possessing the same alleles as the B6 or D2 strains were used. Sufficient data to test two of the five candidate QTL were available. Of these, one was strongly supported as a site affecting brain weight-the D7rp2 region of chromosome 7. For the brain to body weight ratio, four chromosome regions emerged as significantly associated in the BXD series, but none were amenable to testing due to a lack of allelic information for the standard inbred strains. However, two of these regions showed highly significant associations (p < 0.001, single test) that merit consideration as QTL sites for future testing. These two are the Hba region on chromosome 11 and the D17Tu7 region on chromosome 17. The genetic correlation between brain and body weight was low (r = 0.28), indicating that these two traits are largely genetically independent in the BXD RI series.