Final answer:
The majority of chromosomal abnormalities arise during nondisjunction. Nondisjunction occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis.
Step-by-step explanation:
The majority of chromosomal abnormalities arise during nondisjunction. Nondisjunction occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis. This can result in gametes with too many or too few chromosomes, leading to disorders in chromosome number (aneuploidy).