Which of the following is an example of aneuploidy?

• A. Deletion of a single base pair
• B. Trisomy 21 (Down syndrome)
• C. Inversion of a chromosome segment
• D. Translocation between chromosomes

Answer: (B)

This question tests understanding of aneuploidy, a numeric abnormality where the chromosome number is not the normal diploid set. Trisomy 21 has three copies of chromosome 21, giving an extra chromosome in the cell and resulting in Down syndrome. That’s what makes it aneuploidy—a change in chromosome number rather than a change in DNA sequence or in the structure of a chromosome.

The other scenarios involve different kinds of mutations that don’t alter chromosome count: deleting a single base pair is a point mutation affecting a gene’s sequence; inverting a chromosome segment changes the order of genes but not how many copies exist; and translocating between chromosomes rearranges material without changing the total number of chromosomes. So the three-copy chromosome scenario best exemplifies aneuploidy.