From Scientific American:
At first glance identical twins seem, well, identical. In fact many of these sibling pairs show minor physical variations and differences in characteristics such as susceptibility to disease. Just what causes these dissimilarities is unclear. But a new report further suggests that epigenetic factors–that is, differences in how the genome is expressed–could responsible. Environmental factors, including smoking habits, physical activity levels and diet, can influence epigenetic patterns and may help explain how the same genotype can be translated in different ways, the scientists say.
More here.