A man makes 0 units of an enzyme due to homozygous recessive genes on chromosome 21. His wife makes 100 units of the enzyme due to a heterozygous pair of genes on chromosome 21. They have a son with trisomy 21 who makes 200 units of the enzyme.

a. In which parent did the nondisjunction occur?

b. Did the nondisjunction occur in meiosis I or meiosis II (explain your answer)?