What genetic condition is caused by an extra copy of chromosome 21?

Down syndrome is the genetic condition most commonly caused by an extra copy of chromosome 21.

The usual form is trisomy 21, meaning cells have three copies of chromosome 21 instead of two. It generally results from nondisjunction, an error during the formation of an egg or sperm. Less commonly, chromosome 21 material is attached to another chromosome, or only some cells carry the extra copy, a situation called mosaicism.

Down syndrome is associated with characteristic physical features, intellectual disability of varying degree and increased chances of certain medical conditions. People with Down syndrome are individuals with different abilities and health profiles; the effects are not identical in every person.

Edwards syndrome involves an extra chromosome 18, while Turner syndrome usually involves one X chromosome. These conditions are therefore not interchangeable, even though all involve chromosome-number differences.