When the “true” model involves only additive and completely dominant effects at the two candidate loci, the results for the untransformed symptom counts (Test with equal item parameters, Table 6) provide strong support for complex non-additive effects, especially epistatic interactions and G × E interaction. Transformation makes matters worse by strengthening support for epistatic interaction between the candidates. Scores on a test with uniformly distributed difficulties (Test with variable item parameters, Table 6) also suggest some epistasis but provide no hint of G × E. Dichotomizing the scale makes it virtually impossible to say anything certain about genetic architecture in the two-locus case.
