Practice question

During meiosis I, homologous chromosomes normally disjoin to opposite poles at anaphase I. Nondisjunction occurs when a bivalent fails to separate, so both homologs migrate to one pole and none to the other. Consequently both secondary oocytes or spermatocytes become aneuploid. After meiosis II, sister chromatids separate normally, but all four resulting gametes carry unbalanced numbers: two gametes are n+1 disomic and two are n-1 nullisomic for that chromosome. Subsequent fertilization produces trisomic or monosomic zygotes with characteristic syndromes and developmental failure.