A mouse gives birth to identical-twin sisters. One has brown fur and will grow up to be lean and healthy; the other has yellow fur and is destined to be obese and prone to disease. How can two mice sharing exactly the same DNA become so different? The answer lies in the epigenome, a kind of second genome that all animals have, including humans. The epigenome dictates which genes in the genome are turned on and which are not, a process that can differ even between identical twins. In this audio slide show, see how this process works in mice and what the implications are for our own health and that of our children and even grandchildren.—Rima Chaddha

Click on the image to watch this fascinating documentary ~