If a diploid cell undergoes meiosis and produces two gametes that are normal, and one with n - 1 chromosomes, and one with n + 1 chromosomes, what type of error occurred?

Nondisjunction in meiosis II

Explanation:

Nondisjunction is the condition which occurs during the cell division stages in which the homologous pair of chromosomes or the sister chromatids fails to separate during the anaphase in meiosis.

When nondisjunction occurs during anaphase I of meiosis it results in the two cells with monosomy (n-1) and two cells with trisomy (n+1) whereas when nondisjunction occurs during meiosis II, then it results in the formation of two normal cells, one with monosomy (n-1) and one with trisomy (n+1).

In the given question, since the two normal, one n-1 and one n+1 condition are present therefore the nondisjunction occurs in meiosis II.