Which genetic disorder is characterized by the presence of an extra chromosome 21?

Down syndrome is characterized by the presence of an extra chromosome 21, a condition also known as trisomy 21. This genetic anomaly occurs when there is an error in cell division during the formation of reproductive cells (eggs or sperm), leading to an individual having three copies of chromosome 21 instead of the usual two.

The extra genetic material interferes with the normal course of development, causing the characteristics associated with Down syndrome, which may include distinct facial features, developmental delays, and various health concerns. The specific identification of an extra chromosome 21 is what makes Down syndrome unique among genetic disorders, whereas the other mentioned disorders are caused by different chromosomal abnormalities or gene mutations. Turner's syndrome involves a missing X chromosome, cystic fibrosis is caused by mutations in the CFTR gene, and Huntington's disease is related to a mutation in the HTT gene.