Trisomy-21 is popularly known as what?

Trisomy-21 is popularly known as Down syndrome. It is a genetic condition caused by the presence of all or part of an extra copy of chromosome 21, giving cells three copies of that chromosome instead of the usual two.

Most cases result from nondisjunction, an error during the formation of an egg or sperm cell that leaves chromosome 21 paired incorrectly. Less commonly, Down syndrome results from a translocation involving chromosome 21 or mosaicism, in which only some cells carry the extra chromosome. These forms can produce different clinical patterns.

Down syndrome is associated with characteristic physical features, developmental differences, and an increased likelihood of certain medical conditions, including congenital heart defects and hearing or vision problems. The range of abilities and health needs varies widely from person to person.

The condition is named after the British physician John Langdon Down, who described its characteristic features in the 19th century. French geneticist Jérôme Lejeune and colleagues identified the extra chromosome in 1959. A common mistake is to treat “trisomy 21” and “Down syndrome” as different diagnoses: they are two names for the same condition.