A major revision of this text. Since the previous edition was published, there have been substantial advances in understanding of the scientific basis of human genetics, and clinical genetics has finally emerged as a separate clinical specialty. The book retains its distinctive system of numbered sections. There are 162 of which 33 are completely new and 47 are very substantially rewritten. Previous editions have gained a reputation for simple exposition of complex and difficult subjects. In this revision Dr Pembrey has made every effort to carry on this tradition.