What chromosomal disorder involves three copies of chromosome 21?

Down syndrome is a chromosomal disorder that is characterized by the presence of an extra copy of chromosome 21, resulting in three copies instead of the usual two. This additional genetic material alters the course of development and leads to the various physical and intellectual disabilities commonly associated with the condition. The presence of this extra chromosome, also known as trisomy 21, can occur due to nondisjunction during cell division, specifically during meiosis, when the chromosomes do not separate properly. This results in gametes that have an abnormal number of chromosomes, which can lead to Down syndrome if such a gamete contributes to the formation of an embryo.

Klinefelter syndrome, Turner syndrome, and Triple X syndrome involve different chromosomal abnormalities. Klinefelter syndrome results from an extra X chromosome in males, Turner syndrome is the presence of a single X chromosome in females, and Triple X syndrome involves females having an extra X chromosome, none of which pertain to an extra chromosome 21. Therefore, Down syndrome is the distinct disorder that specifically involves three copies of chromosome 21.