Biology stands on the brink of a shift in the understanding of inheritance. The discovery of epigenetics – hidden influences upon the genes – could affect every aspect of our lives. At the heart of this new field is a simple but contentious idea – that genes have a 'memory'. That the lives of your grandparents – the air they breathed, the food they ate, even the things they saw – can directly affect you, decades later, despite your never experiencing these things yourself. And that what you do in your lifetime could in turn affect your grandchildren.