Journal of Medical Genetics (1974). 11, 50. An Analysis of the Break Points of Structural Rearrangements in Man

Summary. The distribution of the points of breakage and reunion of a series of 58 Robertsonian translocations, 53 reciprocal translocations, and 10 inversions is described. An excess of 13/14 and 14/21 rearrangements was found among the Robertsonian translocations, this excess being independent of the method of ascer-tainment of the proband. The distribution of break points between chromosome arms in the reciprocal translocations, with the possible exception ofthe long arms of chromosome 11, was no different from that expected on the basis of their relative lengths. However, within arms there appeared to be an excess of breaks in the terminal regions, an excess of terminal/centromeric translocations where ascertain-ment was through a balanced carrier and a possible excess ofterminal/median trans-locations where ascertainment was through an unbalanced carrier. Nine inversions were analysed and three ofthese involved identical break points on chromosome 8. Possible reasons for the apparent non-randomness of points of breakage and ex-change are discussed and it is concluded that the techniques ofpreparation, methods of observations, and methods of ascertainment all affect the distribution of observed points of breakage and exchange and must therefore be taken into cognizance in any study of chromosome rearrangements in man. The recently developed techniques for banding chromosomes have enabled every chromosome in man to be unequivocably identified and, further-more, because the banding pattern of any particular segment of a chromosome arm retains its charac-teristic morphology when transferred to another part of the complement, structural rearrangements within and between chromosomes can be identified with considerable accuracy. Not only can the chromosome arm involved in the rearrangement be determined but, in many cases, the position of the breakpoints can be localized to a single band. These developments have resulted in two ad-vances in the study of structural abnormalities of the chromosomes. Firstly, they have enabled structural abnormalities previously recognized by simple staining techniques to be accurately classified