The most common chromosome abnormality in humans is trisomy of which chromosome?

Correct answer: C. Chromosome 21 (Down syndrome)

• A. Trisomy 13 (Patau syndrome) is the least common of the three viable autosomal trisomies.
• B. Trisomy 18 (Edwards syndrome) is the second most common viable trisomy but is less common than trisomy 21.
• C. Correct. Trisomy 21 (Down syndrome) is the most common viable chromosomal trisomy and the most common chromosomal disorder in live births, occurring in approximately 1 in 700-800 births.
• D. Sex chromosome abnormalities (Turner, Klinefelter) are also common but trisomy 21 is the most common overall chromosomal abnormality.