Final answer:
An extra copy of two nonhomologous chromosomes is known as partial trisomy, a result of nondisjunction, which can occur in meiosis I or II, leading to an imbalance in chromosome numbers, such as trisomy.
Step-by-step explanation:
An extra copy of two nonhomologous chromosomes is called partial trisomy. This condition is a result of a genetic event known as nondisjunction, which can occur during meiosis I or meiosis II.
Nondisjunction is the failure of replicated chromosomes to separate during meiosis, leading to some gametes with an extra chromosome (or part of a chromosome) and some with a missing chromosome (or part of one). If nondisjunction happens during meiosis I, the homologous chromosomes do not separate properly, resulting in two gametes with two copies of the chromosome, and two gametes lacking that chromosome. Whereas nondisjunction in meiosis II involves the failure to separate sister chromatids, leading to one gamete with two copies of the chromosome, and one gamete lacking it.