Quantitative complementation test. (a) Mating design of a four-way population among the B6.129P-Ly75tm/Mnz/J (KO) strain knocked out for the Ly75 gene, its background C57BL/6 J (B6J) strain, the SR24 subcongenic strain carrying the Pbwg1.5 QTL (see Supplementary Fig. S1) and its background C57BL6/JJcl (B6JJcl) strain. The former two strains have all of the same chromosomes (blue vertical bars) except for the congenic region of the129P2/OlaHsd strain in which alleles at some loci around the Ly75 locus may be different between KO and B6J strains. The latter two strains have the same chromosomes (red vertical bars) except for the subcongenic region in which alleles at some loci may be different between SR24 and B6JJcl strains. In the four-way cross population, four possible genotypes are segregating on a uniform genetic background. No statistical interaction between KO and QTL alleles means that the Ly75 locus is not equal to the QTL, whereas a significant interaction indicates that these loci are the same. (b–d) Weights of white fat pads, unadjusted for body weight at 16 weeks of age. (e–g) Adjusted weights of white fat pads, adjusted for the body weight. The overall mean of adjusted weights for all genotypes was set to zero. P values were obtained from two-way ANOVA. Detailed data for all traits measured are shown in Supplementary Table S2.