Which condition is characterized by an extra chromosome 21?

Down syndrome, also known as trisomy 21, is characterized by the presence of an additional chromosome 21. This condition arises due to nondisjunction during cell division, leading to the development of individuals with three copies of chromosome 21 instead of the typical two. This genetic anomaly can result in various physical and intellectual disabilities, as well as distinct facial features and health issues associated with the condition.

In contrast, Marfan syndrome is caused by a mutation in the fibrillin-1 gene and affects connective tissue, while cystic fibrosis is a genetic disorder caused by mutations in the CFTR gene affecting the respiratory and digestive systems. Sickle cell anemia, on the other hand, is a blood disorder resulting from mutations in the hemoglobin gene, leading to red blood cell deformities. Each of these conditions has a different genetic basis and does not involve the extra chromosome 21 associated with Down syndrome.