What is the name of the genetic disorder caused by an extra chromosome 21?

The genetic disorder caused by an extra chromosome 21 is Down syndrome, also called trisomy 21.

Most cases result from an extra full copy of chromosome 21 in every cell, usually because chromosome pairs fail to separate properly during the formation of an egg or sperm. Less common forms include translocation Down syndrome, in which extra chromosome 21 material attaches to another chromosome, and mosaic Down syndrome, in which only some cells carry the extra material.

The condition is named after English physician John Langdon Down, who described it as a distinct clinical condition in 1866. The chromosomal explanation emerged in 1959, when researchers identified the additional chromosome. The older term “mongolism” is now obsolete and inappropriate because of its racialized origin.

Down syndrome is associated with characteristic physical features, developmental differences, and increased risks of conditions such as congenital heart disease, hearing loss, thyroid disease, and Alzheimer disease. Its effects vary widely among individuals. Prenatal screening can estimate likelihood, while diagnostic tests such as amniocentesis can confirm the chromosome pattern.