A biologist observes a human egg cell that contains two X chromosomes. During which of the following meiotic stages did nondisjunction most likely occur to produce this specific gamete?

Core Concept

Nondisjunction can occur in Anaphase I (homologous chromosomes fail to separate) or Anaphase II (sister chromatids fail to separate). In both scenarios, a gamete can end up with two copies of the X chromosome.