Down syndrome is caused by

• A. Monosomy X
• B. Trisomy 21
• C. Trisomy 18
• D. Polyploidy

Answer: (B)

Down syndrome results from having three copies of chromosome 21, called trisomy 21. This extra copy disrupts normal development and is usually due to nondisjunction during meiosis, so the fertilized egg ends up with three copies of chromosome 21. Other scenarios described by the other options involve different chromosomal changes: missing a sex chromosome (monosomy X) is Turner syndrome; an extra chromosome 18 (trisomy 18) is Edwards syndrome with its own distinct features; and polyploidy means having whole extra sets of all chromosomes, which is not compatible with typical human development. The three copies of chromosome 21 specifically explain Down syndrome.