Which chromosome is associated with Down syndrome's extra copy?

Down syndrome, also known as trisomy 21, is caused by the presence of an extra copy of chromosome 21. This genetic condition typically arises from nondisjunction during cell division, where the chromosomes fail to separate properly. As a result, an individual inherits three copies of chromosome 21 instead of the usual two, leading to the various physical and developmental characteristics associated with Down syndrome. Identifying this specific chromosome is crucial for understanding the genetic basis of the condition and informs both diagnosis and potential research into treatment and support for affected individuals.