Final answer:
Chromosomal nondisjunction is the most common cause of chromosomal aneuploidy, which occurs due to the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leading to gametes with abnormal chromosome numbers.
Step-by-step explanation:
The most common cause of chromosomal aneuploidy is chromosomal nondisjunction. Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate properly during meiosis. This improper separation results in gametes that have an abnormal number of chromosomes, which can lead to aneuploidy once those gametes are involved in fertilization.
During meiosis I, if homologous chromosomes do not separate, it leads to gametes that either lack a chromosome or have an extra one. Similarly, if sister chromatids do not separate during meiosis II, similar aneuploid conditions can occur. These errors in chromosome number can result in various disorders or syndromes depending on the specific chromosomes involved.