On the origin of recurrent trisomy 21: determination using chromosomal and DNA polymorphisms

A family is described in which two cases of trisomy 21 occurred in, respectively, a newborn infant and a prenatally diagnosed fetus. Using fluorescent chromosomal polymorphisms, it was established that in both cases the extra chromosome resulted from a first meiotic division error in the mother and that the father contributed the same centromeric region to both children. RFLP-associated probes were used to examine the genetic content of the chromosomes. It was noted that the polymorphism patterns of the chromosomes 21 which both children inherited from their parents were identical for three, but not identical for one of the probes studied. This difference must be the result of recombination. This result is discussed in relation to the suggestion that the increased recurrence rate in mothers with a trisomic child could be due to a reduced recombination rate.