Monosomy refers to

• A. Absence of one chromosome
• B. Extra copy of a chromosome
• C. Two sets of chromosomes
• D. A chromosome duplication

Answer: (A)

Monosomy is when a chromosome from a homologous pair is missing in a cell; in a normal organism, chromosomes come in pairs, so losing one member of a pair creates monosomy. That’s why the description “absence of one chromosome” fits perfectly. The other options describe different situations: an extra copy of a chromosome is a trisomy, having two complete sets is diploidy (or polyploidy if more), and a duplication refers to extra copies of genetic material on a chromosome rather than the loss of an entire chromosome. A real-world example is monosomy X in Turner syndrome, showing how the lack of a chromosome can still be seen in humans.