Systematic analysis of 95 reciprocal translocations of autosomes

Abstract

The statistical analysis of 95 cases of reciprocal translocations involving autosomes detected among about 10,000 patients studied with the R-banding technique gives the following information: An excess of break points exists for chromosome arms 4p, 9p, 10q, 21q, and 22q and a deficiency for 1p, 2p, and 6q. Furthermore, there are relatively more break points in the small arms than in the large arms, when the translocation is ascertained through an unbalanced translocation carrier. Except for chromosome 22, and ascertainment bias explain this non random distribution. An excess of telomeric break points exists in all cases of translocations ascertained through unbalanced carriers, and an excess of centromeric break point exists in the case of 3:1 and 1:3 segregations only. These excesses are also explained by an ascertainment bias. The break points are located usually at the junction of the bands (interfaces). The size of the chromosomal imbalance varies in the ascertainment classes. It is very large in cases ascertained through balanced carriers (at least one break point is far from the telomere), large in cases ascertained through abortion, and relatively moderate in cases ascertained through unbalanced translocation carriers (at least one break point is juxta telomeric). An excess of balanced reciprocal translocations exists in our sample of mentally retarded and malformed children (position effect?). An excess of balanced reciprocal translocations (not involving chromosome 21) exists among the trisomics 21 and their parents (interchromosomal effect?). A large excess of maternal transmission exists in cases of 3:1 segregation of reciprocal translocation. Deleterious effects of the reciprocal translocations are widely known, but their relation to the topologic changes of the chromatids needs further investigation. Thus, it seemed useful to analyze carefully the 95 reciprocal translocations observed among the 9183 patients studied since banding techniques became available in our laboratory. Our intention was to seek a correlation among the localization of break points, the chromosomes or segments there of involved in the rearrangements, and the types of segregation observed in the families ascertained.