The presence of three copies of chromosome 21 is called

• A. Trisomy 21
• B. Monosomy 21
• C. Polyploidy 21
• D. Nondisjunction 21

Answer: (A)

Having three copies of chromosome 21 is a form of aneuploidy called trisomy 21. In normal human cells, chromosomes come in pairs, but when one chromosome is present in three copies, that extra copy can disrupt development and is linked to Down syndrome. Monosomy 21 would mean only one copy of chromosome 21, which is typically not viable. Polyploidy refers to having an extra full set of all chromosomes, not just a single chromosome. Nondisjunction is the error that can cause trisomy 21 by failing to separate chromosomes during meiosis, but the condition itself is called trisomy 21.