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 ascertainment of the proband. The distribution of break points between chromosome arms in the reciprocal translocations, with the possible exception of the 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 ascertainment was through a balanced carrier and a possible excess of terminal/median translocations where ascertainment was through an unbalanced carrier. Nine inversions were analysed and three of these involved identical break points on chromosome 8.
Possible reasons for the apparent non-randomness of points of breakage and exchange are discussed and it is concluded that the techniques of preparation, 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.
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