What genetic condition is characterized by an extra chromosome, typically chromosome 21?

The genetic condition characterized by an extra chromosome, specifically chromosome 21, is Down's syndrome. This condition is known as trisomy 21, where individuals have three copies of chromosome 21 instead of the usual two. This chromosomal abnormality leads to various physical and intellectual disabilities, which can vary widely among affected individuals. Common features associated with Down's syndrome may include specific facial characteristics, developmental delays, and an increased risk of certain medical issues.

Turner syndrome, Klinefelter syndrome, and Edward syndrome involve different chromosomal abnormalities. Turner syndrome is characterized by the absence of one of the X chromosomes in females, Klinefelter syndrome involves males having an extra X chromosome, and Edward syndrome (trisomy 18) involves an extra chromosome 18. These conditions have distinct genetic and phenotypic presentations that differentiate them from Down's syndrome.