Epigenetics

The chemical modifications that regulate how, where, and when specific genes are turned on and off. These processes do not alter the DNA, itself. Environmental factors can play a critical role in these processes. Epigenetics, also called epigenomics, studies how certain epigenetic modifications may be passed on from one generation to the next. The collection of all epigenetic changes in a genome is called an epigenome.

See the article What is epigenetics? for a good overview.