Which chromosomal abnormality involves the absence of one chromosome from the diploid set?

• A. Monosomy
• B. Trisomy
• C. Polyploidy
• D. Nondisjunction

Answer: (A)

Monosomy is when one chromosome from the normal diploid set is missing, so the chromosome count is 2n−1. This creates aneuploidy, an abnormal number of chromosomes, and in humans it most famously appears as monosomy X (Turner syndrome), where there is only one sex chromosome instead of a pair.

The other terms describe different situations. Trisomy means having an extra chromosome, giving 2n+1 (like Down syndrome with an extra chromosome 21). Polyploidy involves more than two complete sets of chromosomes (such as 3n or 4n), not just a missing one. Nondisjunction is the error in which chromosomes fail to separate properly during meiosis, which can produce monosomy or trisomy, but it refers to the mechanism rather than the specific chromosomal abnormality.