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How does an extra copy of chromosome 21 result in Down syndrome?

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BRCA2 tumor suppressor gene on chromosome 13 — human genetic disease Photomicrograph of the *BRCA2* tumor suppressor gene on chromosome 13 of the human genome. Inactivation of this growth-regulating gene is associated with a higher risk of developing breast cancer.

Down syndrome is a congenital disorder caused by the presence of extra genetic material from chromosome 21, a condition known as trisomy 21.1

This condition typically arises from nondisjunction, which is a random, abnormal event during cell division where homologous chromosomes fail to separate properly during meiosis. When a gamete (egg or sperm) containing an extra chromosome 21 is fertilized, the resulting zygote possesses three copies of the chromosome instead of the normal two, leading to a total of 47 chromosomes in the affected cells.123

The presence of this extra genetic material disrupts normal development, resulting in the characteristic physical and cognitive features of Down syndrome, including intellectual disability, distinct facial features, and potential malformations of internal organs such as the heart or kidneys. While the most common form involves all cells having three copies, variations like mosaicism or translocation can also occur.12