Trisomy 21 Down Syndrome Numerical chromosomal disorders are the result of nondisjunction failure of chromosomes to separate normally during cell division Nondisjunction can occur during meiosis(before fertilization) or mitosis (after fertilization). How could you figure out whether nondisjunction for chromosome 21 had occurred during meiosis or mitosis?Numerical chromosomal disorders are the result of Trisomy 21 Down Syndrome nondisjunction = failure of chromosomes to separate normally during cell division Nondisjunction can occur during How could you figure out whether nondisjunction for chromosome 21 had occurred during meiosis or mitosis? or meiosis (before fertilization) mitosis (after fertilization)