A two-way scan reveals additive but no epistatic interactions between membranous VSD modifier loci. Data are fit to models that assume the presence of zero, one, or two main effect loci with or without epistatic interaction. For an epistatic interaction to be significant, a model that includes the interaction term must fit the data better than one that does not. The full model includes two main effect loci and an interaction between them. The interactive model includes an interaction between two loci but no main effect of either. The LOD score of each marker pair is plotted for the interactive and full models above and below the diagonal, respectively. Significant epistatic interactions must satisfy the conditions that (1) Full LOD > 8.51 and (2) Interactive LOD > 6.1 or Fv1 LOD > 6.69; Fv1 LOD scores, which represent a model of a locus that has a main effect and an interaction with another locus that has no main effect, are not shown.