Down syndrome is associated with trisomy of which chromosome?

• A. 8
• B. 13
• C. 21
• D. 18

Answer: (C)

Down syndrome results from an extra copy of chromosome 21 (trisomy 21). The additional genetic material disrupts normal development, leading to the characteristic facial features, hypotonia, and intellectual disability seen in this condition. This is the most common chromosomal cause of Down syndrome, usually arising from nondisjunction during meiosis and resulting in three copies of chromosome 21 in the resulting cells. Other trisomies—such as trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome)—produce different syndromes with distinct, often more severe congenital anomalies and shorter survival, not the typical Down syndrome presentation. Trisomy 8 is not the cause of Down syndrome, though mosaic trisomy 8 can occur and leads to a different set of abnormalities. Occasionally, a translocation involving chromosome 21 can cause Down syndrome, but the presence of extra chromosome 21 material remains the defining factor.