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Note 1. The first two modes in Figure 1 can be fit neatly by a single locus additive model with the substitution of a pair of DBA/2J alleles adding +10,000 cells to the base population of 54,000. The third mode (BXD5 and BXD32) can be fit by considering one or two additional QTLs (probably a positive alleles from the C57BL/6J parental strain) and by assuming significant non-linear epistatic interactions among loci. All 1-locus, 2-locus, and 3-locus models that we explored required effects of +5,000 to +6,000 per DBA/2J allele to obtain a good fit.

Note 2. While the advantages of RI strains for mapping quantitative traits that are subject to substantial non-genetic variance have been clear for many years, RI strains have rarely been used as the principal method to map QTLs. The main obstacle has been that the number of RI strains is usually too small to define QTLs with phenotypes that are normally distributed or controlled by large numbers of factors.