The origin of human trisomy: a study of heteromorphisms and satellite associations.

The origin of the additional chromosome was studied in 45 trisomic-21 Down-syndrome patients. In 17 patients the additional chromosome was maternal, in 2 it was paternal and in the remaining 26 the parental origin could not be determined. Acrocentric chromosome association was studied in parents of Down-syndrome offspring and in parents of spontaneous abortions that were trisomic for an acrocentric chromosome. Parents of trisomic 16 abortuses and parents of triploid and chromosomally normal abortuses were used as controls. No increased association index was found for the specific acrocentric chromosome involved in the trisomy, either for the liveborn or for the aborted trisomics. However, the overall association index of the parent in whom the non-disjunctional event leading to the acrocentric trisomy occurred was increased by comparison with that of the parent in whom non-disjunction did not occur and with that of the controls. The reasons why we consider satellite associations to play an insignificant role in the aetiology of non-disjunction are discussed.