Fig. 2

Interaction models. Odds ratios for different interaction models, depending on a variant at the first SNP only, the second SNP only or at both SNPs. If both SNPs have no marginal effect, i.e. an odds ratio of 1, but an effect if variants are present at both SNPs, the model is called Interaction only. If only one SNP has a marginal effect and the combined effect is larger than the single marginal effect, the SNP without marginal effect is a Modifier SNP. In No interaction both SNPs have a marginal effect but no additional combined effect. In this example both SNPs have an odds ratio of 1.5 and the combined effect is exactly 1.5²=2.25. If both SNPs have a marginal effect and the combined effect is as large as each single marginal effect, the marginal effects do not add up and we call the model Redundant. If both SNPs have a marginal effect and the combined effect is larger than expected by the marginal effects only, it is a Synergistic model