Which chromosomal disorder is caused by an extra copy of chromosome 21?

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

Answer: (A)

An extra copy of chromosome 21 causes Down syndrome. This condition, known as trisomy 21, usually results from nondisjunction during egg formation, so the embryo has three copies of chromosome 21 in every cell or in a significant portion of cells. The extra genetic material affects development, leading to characteristic facial features, mild to moderate intellectual disability, and potential health issues such as heart defects. The other options describe different conditions: Klinefelter's syndrome involves an extra X chromosome in males (XXY); spina bifida is a neural tube defect not caused by an extra chromosome; cleft palate is a birth defect with multifactorial causes, not defined by trisomy 21.