Abstract
A case is presented in which a 4p+/17q- familial balanced reciprocal translocation in the mother produced a son with primary trisomy-21, as well as the structural chromosomal anomaly. A number of similar situations have been reported, suggesting that the two events are related. In practice, this (as well as other direct risks) should be taken into account when counseling those families in which one parent carries a balanced translocation. A hypothesis, based on experiments in Drosophila, has been put forward by Grell to explain the mechanism which links the balanced structural abnormality to an aneuploidy of chromosomes not taking part in the structural change, and this has been extended to similar human situations.