Since the first edition of this book appeared in 1983 considerable changes have taken place in many aspects of medical genetics. These have been very largely due to the application of recombinant DNA techniques. This technology has helped our understanding of the fine structure of genes and is also beginning to unravel the molecular pathology of many inherited disorders. But perhaps of more immediate and practical importance, the technology has introduced novel and precise methods for detecting female carriers of X-linked disorders, presymptomatic cases of dominant disorders of late onset and in the prenatal diagnosis of genetic disease. These changes are reflected in most of the contributions to this new edition and their relevance will be apparent in almost all aspects of the subject.