Epigenetics

Epigenetics refers to how gene expression is influenced by our environment, e.g., everything we breathe, eat and put on our skin, along with physical and mental trauma. These environmental inputs affect which genes turn on and off, and when they do this. External influences primarily work by triggering a cellular process called methylation that affects genetic function but doesn't change genetic structure.

Epigenetic changes are thought to be a part of numerous diseases, such as cancer, asthma and heart disease.

http://www.the-scientist.com/wordpress/wp-content/uploads/2011/06/epigenetics_primer.pdf