Origin of the additional chromosome in Down's syndrome: a study of 20 families.

In 20 families with a Down's syndrome child, heteromorphism of chromosome 21 was sought in the parents and proband. Information on the meiotic division and the parent of origin was given in 16 families, the majority of errors being maternal in origin, predominantly at meiosis I, though the paternal contribution is also appreciable. Probabilities of parental and meiotic division origin are calculated.