What chromosome is affected in Trisomy 21?

Trisomy 21, commonly known as Down syndrome, occurs when there is an extra copy of chromosome 21. This results in three copies of this chromosome, rather than the usual two. The presence of this additional chromosome alters the typical development and functioning of the body, leading to the features and health issues associated with Down syndrome.

The condition can manifest in a variety of ways, including characteristic facial features, developmental delays, and an increased risk for certain medical conditions. Identifying the specific chromosome affected is crucial for genetic counseling and understanding the implications for individuals and families impacted by the condition. Thus, chromosome 21 is the correct answer, as it is directly linked to the diagnosis of Trisomy 21.