Which chromosomal condition results from an extra copy of chromosome 21?

• A. Klinefelter's Syndrome
• B. Spina Bifida
• C. Cleft Palate
• D. Down Syndrome

Answer: (D)

An extra copy of chromosome 21 causes Down syndrome, a condition known as trisomy 21. Normally, autosomes come in pairs, but when chromosome 21 is present in three copies, gene dosage on that chromosome increases across many genes, leading to the developmental and physical features associated with Down syndrome. This extra copy most commonly arises from nondisjunction during egg formation, and the risk increases with maternal age.

The other conditions listed involve different kinds of birth defects or chromosomal arrangements that do not involve three copies of chromosome 21. Klinefelter’s syndrome involves a sex chromosome configuration (such as XXY); spina bifida is a neural tube defect; cleft palate is a facial structural abnormality. Therefore, the condition produced by an extra copy of chromosome 21 is Down syndrome.