To facilitate ease of study, this edition is divided into three sections. The first outlines the basic principles and adheres as far as possible to the recommendations of the Royal College of Physicians working party on teaching genetics to medical students. In the second section, the horizons of the subject are broadened to show how genetics transcends traditional medical boundaries and is now making a major impact in all areas of medical science. The chapters of the concluding section focus on the practical applications of these new discoveries and how these are being implemented on a widepsread basis through the newly developed discipline of clinical genetics.