The study of 151 reciprocal translocations associated with abnormal probands shows that the mode of imbalance at birth is determined by the nature of the involved chromosomes and by the position of the breakpoints. For each of the three modes (adjacent-1, adjacent-2, and 3:1) there is a corresponding pachytene diagram, so that for each translocation variety it is possible to predict the most probable mode of imbalance. The determining factor is the relative length of the different branches of the cross formed by the tetravalent. However, some heterochromatic regions (9qh, short arms of acrocentric chromosomes) and possibly R-negative regions have a minor role. The factors involved in these mechanisms seem to be the selection and the chiasma position; their respective roles are discussed.