How does aneuploidy change the number of chromosomes in a cell?

Aneuploidy is a condition in which a cell contains an abnormal number of chromosomes that is not an exact multiple of the haploid number. This deviation changes the total chromosome count in a cell, typically resulting in either an extra chromosome or a missing one.1234

The primary mechanism behind this change is nondisjunction, which occurs when homologous chromosomes or sister chromatids fail to separate properly during meiosis or cell division. When a gamete resulting from this error unites with a normal gamete during fertilization, the resulting zygote possesses an unequal distribution of chromosomes.245

For example, in trisomy—a common form of aneuploidy—an individual carries three copies of a specific chromosome instead of the typical two, leading to a total of 47 chromosomes in human cells rather than the standard 46. Conversely, other forms of aneuploidy may involve the loss of a chromosome. These genomic imbalances often result in significant developmental or physiological consequences.42