What is the genetic condition caused by an extra chromosome 21?

The condition caused by an extra chromosome 21 is known as Down syndrome. This genetic disorder, also referred to as trisomy 21, occurs when there is an additional copy of chromosome 21 in a person's genetic material, leading to a total of three copies instead of the normal two. This extra genetic material alters the course of development and is associated with characteristic physical features, intellectual disability, and a variety of health issues.

Individuals with Down syndrome often exhibit distinct facial characteristics, such as a flat facial profile, slanted eyes, and a single transverse palmar crease, among other traits. Moreover, the presence of the extra chromosome can result in various degrees of cognitive impairment and increase the risk of certain medical conditions, including heart defects and gastrointestinal issues.

Turner syndrome, Klinefelter syndrome, and Edwards syndrome are caused by different chromosomal variations and do not involve an extra chromosome 21. Turner syndrome is characterized by a missing or incomplete X chromosome in females, Klinefelter syndrome involves an extra X chromosome in males, and Edwards syndrome is caused by an extra chromosome 18. These conditions have distinct clinical features and implications, further differentiating them from Down syndrome.