Which of the following is a subtype of Down Syndrome characterized by an extra copy of chromosome 21 due to improper separation?

• A. Trisomy 21
• B. Translocation
• C. Mosaicism
• D. Deletion of chromosome 21

Answer: (A)

The key idea is how Down syndrome can arise from extra genetic material on chromosome 21. When improper separation during cell division (nondisjunction) occurs in the formation of the egg or sperm, a gamete may contribute two copies of chromosome 21. After fertilization, the resulting zygote has three copies of chromosome 21 in all cells—this is Trisomy 21, the most common form of Down syndrome.

This differs from other mechanisms: translocation Down syndrome happens when a piece or fragment of chromosome 21 is attached to another chromosome, so the person may have a total chromosome count that looks normal but carries extra chromosome 21 material. Mosaic Down syndrome occurs when nondisjunction happens after fertilization, producing some cells with three copies of chromosome 21 and others with the usual two. Deletion of chromosome 21 involves loss of genetic material, not an extra copy, and does not describe Down syndrome.

So the subtype defined by an extra copy of chromosome 21 due to improper separation is Trisomy 21.