Hum. Genet. 45,259--282 (1978) © by Springer-Verlag 1978 Systematic Analysis of 95 Reciprocal Translocations of Autosomes A. Aurias, M. Prieur, B. Dutrillaux*, and J. Lejeune** Institut de Prog6n~se, 15 rue de l'Ecole de M6decine, F-75006 Paris, France Summary. 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: 1. An excess of break points exists for chromosome arms 4p, 9p, 10q, 21q, and 22q and a deficiency for lp, 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, an ascertainment bias explain this non random distribution. 2. An excess of telomeric break points exists in all cases of translocations ascertained through unbalanced carriers, arid 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. 3. The break points are located usually at the junction of the bands (inter- faces). 4. 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). 5. An excess of balanced reciprocal translocations exists in our sample of mentally retarded and malformed children (position effect?). 6. An excess of balanced reciprocal translocations (not involving chromo- some 21) exists among the trisomics 21 and their parents (interchromosomal effect?). 7. A large excess of maternal transmission exists in cases of 3 : 1 segregation of reciprocal translocation. * Maitre de Recherche au CNRS ** To whom offprint requests should be sent 0340-6717/78/0045/0259/$ 04.80