GENE EFFECTS ON A QUANTITATIVE TRAIT: TWO‐LOCUS EPISTATIC EFFECTS MEASURED AT MICROSATELLITE MARKERS AND AT ESTIMATED QTL

Most evolutionarily and agriculturally important traits are affected by many genes (quantitative trait loci, or QTL) of relatively small effect. Usually the genetics of these traits are examined by indirect statistical analysis of the covariance among relatives, rather than by direct analyses. We use new analytical and molecular techniques to examine nonadditive interactions of microsatellite markers and estimated QTL that influence adult body weight in mice. Offspring of a cross between a large inbred mouse strain (LG/J) and a small inbred strain (SM/J) were intercrossed to form a segregating F2 generation. Using 76 microsatellite markers and 19 estimated QTL, we estimate gene‐level epistasis and population‐level epistasis for body weight at 10 weeks for 534 F2 mice. Significant epistasis was found for large numbers of the two locus comparisons using both markers and previously detected QTL. There are many genes segregating for adult body weight in this cross and many of these genes appear to interact epistatically. The discovery of potentially extensive epistasis has important implications for evolutionary models.