Odds ratio of recurrence (ORR) as a function of the gene–environment interaction coefficient (I) for varying frequencies of predisposing genotype(s) (fG), varying genotypic relative risks (RRG) and for a recessive and a dominant genetic model. Fixed parameters: disease prevalence, fD=0.1; frequency of exposure, fE=0.2; exposure relative risk, RRE=1; sibling correlation for the environmental factor, CE=0.