This is an example of additive effects of genetic and environmental risk factors. The difference in the absolute risks (or incidence rates) for susceptible individuals with and without the exposure is 6. The difference in the absolute risks for non-susceptible individuals with or without the exposure is also 6. So the effect of the exposure is additive whether or not individuals are genetically susceptible. Note also that the differences in the odds ratio for the S+E+ and S+E- is 2, as is the difference between the S-E+ and S-E- groups.