Down syndrome is a genetic defect that occurs as the result of which chromosomal event?

• A. An extra pair of chromosomes.
• B. A sperm that contains 24 chromosomes.
• C. A separation of chromosome 21.
• D. A triplication of chromosome 21.

Answer: (D)

Down syndrome occurs when there is an extra copy of chromosome 21 in the body's cells. This means three copies of chromosome 21 are present instead of the usual two, a situation called trisomy 21. That additional genetic material disrupts development, leading to the characteristic features of Down syndrome. A triplication of chromosome 21 conveys the same idea: three copies of that chromosome rather than two. The other options describe different scenarios that don’t specifically involve having an extra copy of chromosome 21, so they don’t explain Down syndrome as accurately.