To investigate the origin of nondisjunction, 200 human oocytes that had failed to be fertilized dur… Show more To investigate the origin of nondisjunction, 200 human oocytes that had failed to be fertilized during in vitro fertilization procedures were examined. These oocytes had completed meiosis I and were arrested in metaphase II (MII). The majority had a normal MII metaphase complement, showing 23 chromosomes, each consisting of two sister chromatids joined at a common centromere. The remaining oocytes all had abnormal chromosome compositions. Surprisingly, when trisomy was considered, none of the abnormal oocytes had 24 chromosomes. • Show less