nondisjunction

The failure of paired (homologous) chromosomes to separate during anaphase 1 of meiosis. As a consequence, one daughter cell, and hence the two gametes formed from this, will receive both homologues. By corollary, the other daughter cell, and the gametes formed from this, will be deficient for the chromosome in question. Cells containing abnormal numbers of chromosomes are called aneuploids (see aneuploidy ). See also monosomic , trisomic.