Additive effect of the Aia3 and Nba2 loci on the production of Coombs antibodies in B6 × (NZB × B6.Yaa)F1 BC male mice bearing the Yaa mutation. Coombs antibody activities were determined at 10 months of age. BC mice were subdivided into 4 groups according to genotype combinations at markers Cd22 and Fcgr2, corresponding to the Aia3 and Nba2 loci, respectively. N/B indicates NZB/B6 heterozygous genotype; B/B, B6/B6 homozygous genotypes. Each symbol represents an individual animal in each group: mice heterozygous at both loci (n = 37), mice heterozygous at only Cd22 (n = 40), mice heterozygous at only Fcgr2 (n = 32), and mice homozygous for the B6 alleles at both loci (n = 35). Mean values are indicated by horizontal lines. Significant differences between the groups are shown by asterisks (*P < .01, **P < .005, ***P < .001).