What condition is characterized by the presence of an extra copy of chromosome 21?

Down Syndrome is characterized by the presence of an extra copy of chromosome 21, leading to the condition known as trisomy 21. This genetic anomaly results in a total of three copies of this chromosome instead of the usual two. People with Down Syndrome often exhibit distinct physical features, developmental delays, and an increased risk of certain medical conditions.

In contrast, Turner Syndrome is caused by the complete or partial absence of one of the X chromosomes in females; Klinefelter Syndrome occurs in males who have an extra X chromosome (XXY instead of the typical XY); and Fragile X Syndrome is a genetic condition caused by a mutation on the FMR1 gene located on the X chromosome, affecting intellectual and developmental abilities. Each of these conditions has unique chromosomal and phenotypic characteristics that differentiate them from Down Syndrome.