viii From discussions with our colleagues, we know that they recognize the problems and worry about them, but simply do not have the time to thoroughly study the highly specialized genetic literature available. This book is an attempt to fill this void. We have made an effort to keep it as short and clear as possible and to limit it to the important and most frequent genetic abnor malities. In particular, we have tried to take into consid eration the difficulties of the average student in under standing genetic logic and to eliminate the most common errors. This guide is not designed to provide more than basic information. No reader will arise from the study of this volume as an expert genetic counselor. That requires, in this as in all other sciences, knowledge of the highly spe cialized literature as well as extensive experience. Some geneticists therefore take the position that the general practitioner (or specialist in any other field of medicine) cannot possibly give proper genetic counsel to his patients. Because he is not a genetics expert, he should, without exception, refer all such cases to the geneticist. This point of view would condemn this guide as potentially more harmful than helpful in that it might increase the cases of well-meaning error as well as encouraging those who are not competent in this field to deal with problems which are beyond their capacity. We, obviously, do not share this pessimistic standpoint.