Health Explained: Epigenetics
14 March 2012 Last updated at 17:13 GMT
Professor Tim Spector of The Department of Twin Research & Genetic Epidemiology at King's College London, explains what epigenetics are.
Using the example of identical twins, he shows how genes can only explain so much - and how we can also be affected by environmental factors such as what we eat, and the stress we endure.
These other factors can result in changes at the molecular level in all of us throughout our lives.
Stills courtesy of the Science Photo Library