Nondisjunction is the failure for homologous chromosomes to separate during mitosis or meiosis. If nondisjunction happens during mitosis, one cell would have nothing and the other would be tetraploidy. If nondisjunction happens during meiosis I, all the gametes will be abnormal. The gametes will carry both or neither member of the pair. If nondisjunction happens during meiosis II, two will be normal and two will be abnormal.
When a cell's meiosis I undergoes nondisjunction. As a result, 3 chromosome 13s is in one cell. Then that cell undergoes meiosis and one daughter cell has 2 chromosome 13s. When that gamete fertilizes to make a zygote, 3 chromosome 13s will appear.
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