Which genetic disorder is characterized by the presence of an extra chromosome 21?

Down syndrome is a genetic disorder caused by the presence of an extra chromosome 21, which results in a total of three copies of this chromosome instead of the usual two. This condition is also known as trisomy 21. The additional genetic material affects the individual's development, leading to distinct physical features and varying degrees of intellectual disability.

Individuals with Down syndrome often exhibit certain traits such as a flat facial profile, an upward slant to the eyes, a short neck, and a protruding tongue. They may also have an increased risk of congenital heart defects and other health issues. The presence of the extra chromosome disrupts normal development, which is why Down syndrome is linked to these specific phenotypic characteristics and health concerns.

In contrast, Turner syndrome involves a complete or partial absence of one X chromosome in females, cystic fibrosis is caused by mutations in the CFTR gene affecting the respiratory and digestive systems, and sickle cell anemia results from a mutation in the hemoglobin gene affecting red blood cell shape and function. Each of these conditions has its own distinct causes and symptoms, differentiating them from Down syndrome.