Aneuploidy (an abnormal number of chromosomes) occurs due to one particular erroneous meiotic event called:
a) nondisjunction (chromosomes don't separate).
b) prophase I (replicated chromosomes condense).
c) metaphase I (paired homologs align down equator).
d) anaphase II (centromeres part).
e) crossing-over (swapping genetic material between chromosome homologs).
Answer: a) nondisjunction (chromosomes don't separate).