In a provocative report from the frontiers of biology, NOVA explores new findings that call into question the long-held belief that all inherited traits are passed on by our genes. The fast-growing field of epigenetics investigates hidden influences that could affect not only our health today but that of our descendants far into the future. It now seems that our environment makes small chemical changes to our DNA without affecting the gene's overall makeup. To put it another way, epigenetics adds to our DNA another layer that acts as a control system of "switches." Experiential factors such as nutrition or stress may trigger these switches and turn genes on or off. These subtle changes can then be "remembered" and passed on from generation to generation, altering inherited traits. This means the lives of our grandparents - the air they breathed, the food they ate, even the things they saw - could have directly affected us, and that what we do could, in turn, affect our grandchildren. NOVA explores this fascinating new idea, interviewing top scientists in the field and following what could be a paradigm shift in the way we think about inheritance and genes.