The clinical side of cytogenetics.

In 1956 the correct number of chromosomes was established in man, and in 1959 the well-recognized syndromes of Down, Turner and Klinefelter were shown to be associated with chromosomal anomalies. During the following 10 years more than 100 chromosomal anomalies were discovered by what now appear to have been rather crude and undiscriminating techniques, as reflected in the repeated attempts to standardize the human karyotype, first at Denver in 1960, then in London in 1963 and in Chicago in 1966. At the Second Denver Conference in 1970 a dramatic change took place as a result of the introduction of a technique by which every individual chromosome could be distinguished; by the time the next Conference on Standardization in Human Cytogenetics was held, in Paris in 1971, additional techniques had been developed that could distinguish not only every chromosome but also different regions of each chromosome. We are now able to define normal variants reliably, identify extra chromosomes involved in abnormalities accurately, recognize small defects which were missed previously and map structural defects precisely by tracing exchange segments of chromosomes. Cytogenetics should be used by the clinician to establish a diagnosis as a guide to a rational plan of management, as a guide to prognosis and genetic counseling and for monitoring pregnancies in people at increased risk of having children with defects associated with chromosomal anomalies.