IT HAS become standard practice to study the chromosomes of parents of children with Down's syndrome before giving genetic counseling. In some families the presence of a balanced translocation greatly increases the risk of having another similarly affected child. The chance of an unbalanced karyotype and, therefore, an affected individual can be estimated from the normal segregation ratios. Before giving counseling, however, it is important to realize that the existence in a parent of a balanced translocation can result in an abnormal child as the result of a chromosome abnormality other than an unbalanced translocation.
In phenotypically normal individuals balanced translocations can at times interfere with meiosis in such a way as to cause nondisjunction.1-4 The resulting offspring carry the balanced translocation and, in addition, are trisomic for another chromosome,2-4 or in the case of the X chromosome, they are monosomic.1 We are reporting our findings in