What is the failure of chromosomes to separate properly during meiosis, often resulting in aneuploidy?

Nondisjunction refers to the process where chromosomes fail to separate accurately during meiosis, leading to gametes that contain an abnormal number of chromosomes. This improper separation can result in aneuploidy, which is a condition where a cell has either an excess or a deficit of chromosomes. For instance, if an egg with an extra chromosome is fertilized, it can lead to conditions like Down syndrome, which is caused by having an extra copy of chromosome 21.

Other concepts, such as chromosomal aberrations and segregation errors, may relate to changes or mistakes in chromosomes but do not specifically capture the mechanism of failure to separate during meiosis as nondisjunction does. Disjunction generally refers to the normal separation of chromosomes, making nondisjunction the most accurate term to describe the failure in this context. Thus, nondisjunction is the correct answer since it precisely describes the mechanism that leads to aneuploidy due to missegregation of chromosomes during meiosis.