What is trisomy 21?

What is trisomy 21?

• A. A chromosomal disorder caused by a deletion on chromosome 21.
• B. A genetic disorder caused by a missing 21st chromosome.
• C. An abnormal number of 21st chromosomes due to a translocation.
• D. A genetic disorder caused by the presence of an extra 21st chromosome.

Answer: (D)

Trisomy 21 means there are three copies of chromosome 21 in the body's cells instead of the usual two. This extra copy increases the amount of gene products from chromosome 21, leading to the features of Down syndrome. It most often occurs when chromosomes don’t separate properly during egg or sperm formation (nondisjunction), so the embryo ends up with three copies of chromosome 21. The other options describe a deletion, a missing chromosome, or a rearrangement—none of which define trisomy 21. Translocation can sometimes cause Down syndrome, but the defining idea of trisomy 21 is the presence of an extra chromosome 21.

Trisomy 21 means there are three copies of chromosome 21 in the body's cells instead of the usual two. This extra copy increases the amount of gene products from chromosome 21, leading to the features of Down syndrome. It most often occurs when chromosomes don’t separate properly during egg or sperm formation (nondisjunction), so the embryo ends up with three copies of chromosome 21. The other options describe a deletion, a missing chromosome, or a rearrangement—none of which define trisomy 21. Translocation can sometimes cause Down syndrome, but the defining idea of trisomy 21 is the presence of an extra chromosome 21.