Common variations associated with disease susceptibility - pharmacogenetics
It has been known for a long time that common genetically determined variations which are not disease-causing in themselves can lead to disease in response to environmental exposure to substances to which other individuals do not respond adversely.
The first examples of this were discovered in the early 1950s. It seems very possible there will be a rush of new examples in the next few years as we enter the ‘post-genome’ era.
http://www.ucl.ac.uk/~ucbhjow/b241/biochemical.html