This BBC video clip first reviews the basic mechanisms of inheritance. Next, it presents evidence that initially confounded researchers working on The Human Genome Project. How can the human species have fewer genes in its genome than do plants? How can two very rare, and very different genetic disorders—Prader-Willi syndrome and Angelman syndrome—be caused by exactly the same deletion of genes on chromosome 15? The answers to these questions led to an understanding of genetic imprinting, and the emerging field of epigenetics. Genetic inheritance entails more than just which genes are inherited. It involves which genes are expressed and which are silenced.