Applying a multiplicative model, we found no significant interaction between TTC12-rs10502172 and CHRNA3-rs1051730 (Age 14: Wald-Chi-square=1.26, df=2, P=0.53; Age 31: Wald-Chi-square=0.81, df=2, P=0.67). In adolescence, the combined effect of these two loci seemed additive (Figure 2). Carriers of three/four risk alleles (20% of the population) had almost 3-fold increased odds of smoking regularly and almost 2.5-fold increased odds of being occasional-smokers as compared to subjects with no risk alleles (9% of the population). Carriers of 1–2 risk alleles had an intermediate risk pattern.
