Since the first edition of this book in 1989 there have been enormous changes in clinical genetics, reflecting the knowledge generated from the tremendous advances in molecular biology, culminating in the publication of the first draft of the human genome sequence in 2001, and the dissemination of information via the internet. The principles of genetic assessment and the aims of genetic counselling have not changed, but the classification of genetic disease and the practice of clinical genetics has been significantly altered by this new knowledge. To interpret all the information now available it is necessary to understand the basic principles of inheritance and its chromosomal and molecular basis. Recent advances in medical genetics have had a considerable impact on other specialties, providing a new range of molecular diagnostic tests applicable to many branches of medicine, and more patients are presenting to their general practitioners with concerns about a family history of disorders such as cancer. Increasingly, other specialties are involved in the genetic aspects of the conditions they treat and need to provide information about genetic risk, undertake genetic testing and provide appropriate counselling. All medical students, irrespective of their eventual career choice therefore need to be familiar with genetic principles, both scientific and clinical, and to be aware of the ethical implications of genetic technologies that enable manipulation of the human genome that may have future application in areas such as gene therapy of human cloning. The aim of this third edition of the ABC is therefore to provide an introduction to the various aspects of medical genetics for medical students, clinicians, nurses and allied professionals who are not working within the field of genetics, to generate an interest in the subject and to guide readers in the direction of further, more detailed information.