Advanced Placement (AP) Biology Practice Exam

What genetic condition is characterized by three copies of the 21st chromosome?

1. Monosomy

2. Trisomy 21

3. Polyploidy

4. Aneuploidy

The correct answer is: Trisomy 21

Trisomy 21 is a genetic condition characterized by the presence of three copies of chromosome 21 instead of the usual two. This condition is more commonly known as Down syndrome, which can result in various physical and intellectual disabilities. The reason trisomy occurs is due to a nondisjunction event during gamete formation, where chromosomes fail to separate properly. As a result, when fertilization occurs, the embryo has an extra chromosome 21. This genetic anomaly reflects the concept of trisomy, indicating that there are three instances of a particular chromosome rather than the normal pair. While other choices may refer to genetic conditions or chromosomal alterations, they do not specifically denote the presence of three copies of chromosome 21. Monosomy refers to having only one copy of a chromosome instead of the usual two, polyploidy involves having more than two complete sets of chromosomes, and aneuploidy refers to an abnormal number of chromosomes (which can include either more or fewer than the normal diploid number). Therefore, the specific nature of having three copies of the 21st chromosome is accurately described by trisomy 21.