Although the most frequent forms of Down syndrome are caused by a random error, nondisjunction of chromosome 21, Down syndrome occasionally runs in families. The cause of this form of familial Down syndrome is _.

A) an inversion involving chromosome 21
B) too many X chromosomes
C) a maternal age effect
D) a translocation between chromosome 21 and a member of the D chromosome group
E) a chromosomal aberration involving chromosome 1