Aneuploidy refers to

• A. Normal chromosome number
• B. An abnormal number of chromosomes
• C. A mutation in a single gene
• D. A change in chromosome structure

Answer: (B)

Aneuploidy is about chromosome number, not chromosome structure or single-gene changes. It means a cell has more or fewer chromosomes than the normal set for the species. In humans, that often happens when chromosomes don’t separate properly during meiosis (nondisjunction), producing gametes with an abnormal number of chromosomes. If such gametes are involved in fertilization, the resulting embryo has an abnormal chromosome count. This is different from a mutation in a single gene, which involves a change in the DNA sequence of one gene, or from a change in chromosome structure (like deletions, duplications, inversions, or translocations) where the number is usually still normal. Examples of aneuploidy include Down syndrome (an extra chromosome 21) and Turner syndrome (a missing X chromosome in females).