Which genetic disorder is due to an extra copy of chromosome 21?

Down syndrome is characterized by the presence of an extra copy of chromosome 21, resulting in a total of three copies instead of the usual two. This genetic anomaly occurs due to a nondisjunction event during meiosis, where chromosomes fail to separate properly, leading to a gamete with an extra chromosome. When this gamete unites with a normal gamete at fertilization, the resulting embryo has three copies of chromosome 21. This additional genetic material affects development and can lead to a variety of physical and intellectual challenges associated with Down syndrome, including distinctive facial features, developmental delays, and a higher likelihood of certain health issues.

In contrast, the other options refer to different genetic conditions that are not caused by an extra copy of chromosome 21. For instance, Fragile X syndrome is related to a mutation on the FMR1 gene on the X chromosome, while both 22q11 deletion syndrome and Velocardiofacial syndrome involve deletions of chromosome segments rather than the presence of an extra chromosome.