Which condition is an example of monosomy?

• A. Klinefelter syndrome
• B. Triple X syndrome
• C. Down syndrome
• D. Turner syndrome

Answer: (D)

Monosomy is when there is only one copy of a chromosome in a cell, instead of the usual two. In humans, most autosomal monosomies are not compatible with life, but a single missing sex chromosome can be viable. Turner syndrome is the example here because it involves only one X chromosome (45,X) in females. The other conditions involve extra copies of chromosomes: Klinefelter syndrome has an extra sex chromosome (XXY), Triple X has an extra X (XXX), and Down syndrome has an extra copy of chromosome 21 (trisomy 21). So Turner syndrome is the monosomy among the options.