USA Biology Olympiad (USABO) 2025 – 400 Free Practice Questions to Pass the Exam

Trisomy 21, also known as Down syndrome, is most commonly caused by errors in meiosis, which is the process of cell division that produces gametes (sperm and eggs). During meiosis, chromosome pairs are supposed to segregate evenly so that each gamete receives a single copy of each chromosome. However, if an error occurs, such as nondisjunction, where the chromosomes fail to separate properly, it can lead to gametes with an abnormal number of chromosomes. If such a gamete contributes to fertilization, the resulting zygote will have three copies of chromosome 21 instead of the normal two, resulting in trisomy 21.

This mechanism of failure during meiosis is specific to the formation of gametes and is distinct from processes that might affect somatic cells throughout the body or errors that occur during mitosis, which affects cell division in non-gametic cells. Environmental factors also play a role in the risk of trisomy 21, such as maternal age, but they do not directly cause the chromosomal abnormality. Thus, the association of trisomy 21 with errors during meiosis makes that answer correct, reflecting the primary cause of the condition.