Which condition results from having an extra chromosome 21?

Down's Syndrome, also known as Trisomy 21, results from the presence of an extra copy of chromosome 21. This genetic condition is characterized by various physical and developmental features, including distinct facial traits, increased risk of congenital conditions, and varying degrees of intellectual disability. The presence of three copies of chromosome 21 alters typical genetic functioning and leads to the assortment of characteristics associated with Down's Syndrome.

In the context of genetic disorders, Klinefelter's Syndrome is caused by an extra X chromosome in males, Turner Syndrome involves the absence of one X chromosome in females, and Sickle Cell Anemia is a blood disorder caused by a specific genetic mutation in the hemoglobin gene. Each of these conditions is linked to different chromosomes and genetic mechanisms, making Down's Syndrome uniquely defined by the extra chromosome 21.