Although the most frequent forms of Down syndrome are caused by a random error, nondisjunction of chromosome 21, Down syndrome occasionally runs in families. Which of the following is the cause of this form of familial Down syndrome?
A) an inversion involving chromosome 21
B) a chromosomal aberration involving chromosome 1
C) too many X chromosomes
D) a translocation between chromosome 21 and a member of the D chromosome group
E) a maternal age effect