How does trisomy for chromosome 21 happen?
Trisomy 21 occurs when one of the gametes results from a meiosis in which chromosome 21 nondisjunction occurred during either meiosis I (about 80% of cases) or meiosis II (about 20% of cases), giving rise to two (rather than one) chromosomes 21 in this gamete. At fertilization, the other gamete supplies the third #21. The probability of such nondisjunction occurring during gamete production is low in young adults. For women, however, the risk of nondisjunction occurring during egg production goes up dramatically with age. Figure 8.12 shows the results of a study of all births in Sweden over a two year period.
