Which genetic condition is caused by an extra copy of chromosome 21?

• A. Klinefelter syndrome
• B. Down syndrome
• C. Turner syndrome
• D. Fragile X syndrome

Answer: (B)

Having an extra copy of chromosome 21 results in Down syndrome, a trisomy condition. Humans normally have two copies of each chromosome, but in Down syndrome there are three copies of chromosome 21 in the cells. This extra genetic material affects development and leads to the characteristic features and health considerations associated with the condition. Most often this arises from nondisjunction during egg formation, so the risk increases with maternal age, though other rare mechanisms like translocation can also produce trisomy 21. The other options involve different chromosomal changes: Klinefelter syndrome is an extra X chromosome (XXY) in males, Turner syndrome is a missing X chromosome (monosomy X), and Fragile X syndrome is a gene mutation on the X chromosome, not an extra chromosome.