Answer: D. Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis. This can produce a gamete with two copies of chromosome 21; fusion with a gamete containing one copy gives a zygote with three copies, associated with Down syndrome. A: crossing over exchanges chromosome segments but does not produce an extra copy of chromosome 21. B: a mutation changes DNA sequence, not the number of chromosome 21 copies. C: normal separation gives each gamete one copy of chromosome 21. Remember: nondisjunction can change the number of chromosomes in a gamete.