The London Conference on ‘ The Normal Human Karyotype’ *

The present meeting was called to consider developments since the Denver Conference and to assess the degree to which these may have aided characterization of individual chromosomes or may have revealed inadequacies in the earlier identifications. There was general agreement that the seven groups proposed in the Denver numbering system have stood the test of time, and that little or no uncertainty exists in the group assignment of any chromosome. However, within certain groups identification of individual chromosomes is questionable. The groups are sometimes designated by letters, such as A to G, as well as by ths numbering scheme described in the Denver classification. A comparison of data submitted by different investigators revealed that surprisingly good agreement on relative chromosomal length and arm ratios or centromeric indices can be obtained in different laboratories provided that mitotic figures, clearly delineated and free from irregularities of condensation pattern, are selected for analysis. The dispersion of chromosomal measurements appears to be due to both biological and technical factors and, while some of these variations are random, others appear to be systematically produced, as in the effect of exposure to colchicine which may contract some chromosomes more than others. It is hoped that specific procedures maximizing ease of identification will soon be forthcoming. The new techniques considered included the demonstration of specific chromosomal constrictions and autoradiographic determination of the time at which 3H thymidine is incorporated in specific chromosomes and chromosomal regions. Both of these methods contribute to the characterization of specific chromosomes. However, variability exists in techniques and material ; so that at present neither method permits absolute identification of chromosomes. There was discussion on the use of methods for DNA measurement in each chromosome arm, on electron microscopy, on satellite association, on specific location of certain chromosomes within spread mitotic figures, on physico-chemical separation of chromosomes in suspension, on the u8e of reagents which may characterize chromosome segments and on the ash patterns produced by micro-incineration, It was agreed at Denver that the autosomes should be serially numbered 1 to 22 as nearly as possible in descending order of length. Identification of individual chromosomes is based on size, position of centromere and other morphological features. The sex chromosomes are referred to as X and Y . The most important newly recognized features are so-called secondary constrictionst and differential patterns of incorporation of isotopically labelled thymidine in specific chromosomes as demonstrated by autoradiography. Secondary constrictions are only seen in certain types of preparations and then in a fraction of the cells. Variability is observable in the morphology of certain chromosomes. This is especially well