What is your full potential? Are you held back by your genes? Limited by whatever you inherited from your parents? Is our life’s path set at birth or can we overcome our genetic destiny? We are living in age of genetic testing, engineering and cloning: for a few quid anyone can send off a DNA sample and get results about where they came from, who they are, and what their future holds. It's all pretty confusing but Dr Chris and Dr Xand van Tulleken, TV presenters and identical twins, have lived their whole lives as an experiment to get some answers.