Final answer:
Nondisjunction is the event where all chromosomes fail to separate during meiosis, resulting in all four daughter cells being abnormal.
Step-by-step explanation:
Nondisjunction is the event where all of the chromosomes fail to separate normally during meiosis, resulting in all four daughter cells being abnormal. It can occur during either meiosis I or meiosis II. If homologous chromosomes fail to separate during meiosis I, two gametes will lack that particular chromosome and two gametes will have two copies of the chromosome. If sister chromatids fail to separate during meiosis II, one gamete will lack that chromosome, two gametes will have one copy of the chromosome, and one gamete will have two copies of the chromosome.