The last decade has seen remarkable advances in human ge netics. Once the correct chromosome number of the human genome was ascertained, a wide variety of diseases was rec ognized as due to numerical chromosome anomalies. There followed the discovery that spontaneous abortions are the result of chromosome errors, and specific band patterns of chromosomes allowed identification of minute lesions. The techniques of cell hybridization now allow specific gene assign ment to chromosomes and even to distinct loci on their arms. All this was possible because of the ease with which metaphase...