Gene environment interaction is an influence on the expression of a trait that results from the interplay between genes and the environment. Some traits are strongly influenced by genes, while other traits are strongly influenced by the environment. Most traits, however, are influenced by one or more genes interacting in complex ways with the environment.
Lots of people. assume that we geneticists are only really interested in the genes and we don't think the environment is very important. Well, that's certainly not the case. For most complex diseases like diabetes and cancer, or heart disease, it's an interchange between genes and environment that gives rise to disease. You may be predisposed in a certain way by genetics, but you're probably not going to get the disease unless the environmental trigger is present, too. So this is a hugely important area of current research, to try and understand how the genes and the environment work together and how we can modify the environment for somebody whose genetic susceptibilities indicate that they're at risk.