What is the genetic basis of Down syndrome?

Down syndrome, also known as trisomy 21, is characterized by the presence of an additional copy of chromosome 21. This genetic anomaly typically arises from nondisjunction during cell division, leading to gametes with an abnormal number of chromosomes. When one of these gametes is involved in fertilization, the resulting zygote will have three copies of chromosome 21 instead of the usual two.

This extra genetic material alters the course of development and leads to the distinctive physical features and varying degrees of intellectual disability associated with Down syndrome. The other choices highlight different genetic abnormalities that do not correlate with Down syndrome. For instance, a mutation in a gene sequence might lead to other genetic disorders but not specifically Down syndrome, while a deletion of chromosome 15 is associated with other syndromes like Prader-Willi or Angelman syndrome. An additional X chromosome links to Klinefelter syndrome rather than Down syndrome. Therefore, the presence of an extra copy of chromosome 21 is the hallmark of Down syndrome.