The origin of the extra chromosome 21 in Down syndrome. Studies of fluorescent variants and satelite association in 26 informative families.

Studies of fluorescence and other chromosomal variants were informative in 26 out of 72 families. Maternal nondisjunction was found in 19 and paternal in 7 cases. Satellite association studies of these parents and 94 controls from the same age group showed a highly significant increase in the satellite association index (AI) for chromosome 21 in the parents where the nondisjunctional event had taken place. The AI was also higher for chromosome 14. In addition, the parents who produced the normal gametes had significantly higher AI's for some acrocentrics than the controls. Exogenous factors increasing satellite association cannot be ruled out. The number of 21-21 association was significantly increased in the parents with nondisjunction in meiosis 1. The results indicate that satellite association may play a role in the etiology of Down syndrome.