The table below shows the odds ratios for the outcome of cleft lip and palette in newborns among pregnant women with particular genotypes of a SNP from the TGFa gene (Any A2 genotype compared to only A1 genotypes), additionally stratified by maternal smoking status during pregnancy. The odds ratios displayed below are an example of which of the following? (Choose the one best answer)

Smoking during pregnancy	Genotype	Odds ratio
Heavy smoker	Any A2	5.10
Heavy smoker	Only A1	1.79
Non or light smoker	Any A2	0.68
Non or light smoker	Only A1	1.00 (Reference)

Confounding

Environment-environment interaction

Gene-environment interaction

Gene-gene interaction

None of the above