Which syndrome is caused by trisomy 21?

• A. Turner’s syndrome
• B. Down syndrome
• C. Klinefelter syndrome
• D. Edwards syndrome

Answer: (B)

Having three copies of chromosome 21, or trisomy 21, leads to Down syndrome. The extra genetic material disrupts normal development, causing a characteristic pattern of features such as mild-to-moderate intellectual disability, distinct facial characteristics, and potential heart defects, along with hypotonia in infancy. This condition most often arises from nondisjunction during the formation of eggs or sperm, resulting in a gamete with an extra chromosome that is then present in every cell after fertilization. The other listed syndromes involve different chromosomal changes: Turner syndrome is the loss of one X chromosome in females, Klinefelter syndrome is an extra X chromosome in males, and Edwards syndrome is trisomy 18, which has its own distinct and severe presentation. So trisomy 21 specifically corresponds to Down syndrome.