Rate of recombination of chromosomes 21 in parents of children with Down syndrome.

To test the hypothesis of reduced chiasma frequency causing nondisjunction during meiosis, we examined 34 Down syndrome patients and their parents. Chromosomal polymorphisms and RFLP markers were used to trace the parental origin as well as the frequency of recombination of chromosomes 21. In all but one case, the parental origin and the meiotic stage of nondisjunction could be established by either technique. In 11 cases recombination could be deduced to have taken place during meiosis in the parent who contributed the extra chromosome 21. Because of the underestimation which is inherent in the methods used, these results do not seem to support the chiasma theory.