This book considers the branch of heredity known as "epigenetics" and its implications for a variety of diseases in humans and animals. After background information on the growth in understanding genetics and the mechanisms of the epigenetic control of gene expression, the book moves into its main focus: the gathering body of evidence connecting genetics to a range of significant illnesses, including cancer, autism, Alzheimer's disease, diabetes and others. Areas of uncertainty are stressed as well as the scientific debate concerning the role of environmental factors. The final chapters discuss the implications for society. Extensive notes provide additional details and personal anecdotes.