Down syndrome is a genetic condition caused by the presence of an extra copy of chromosome 21. This additional genetic material affects the way a person's body and brain develop, leading to certain physical and intellectual characteristics. It is named after John Langdon Down, the British doctor who first described the syndrome in 1866.

Individuals with Down syndrome typically exhibit physical features such as almond-shaped eyes, a single crease across the palm, and a flattened facial profile. These characteristics can vary widely, and it's important to note that individuals with Down syndrome are unique individuals with their own distinct personalities, talents, and abilities.

One of the most common forms of Down syndrome is Trisomy 21, where there is an extra copy of chromosome 21 in every cell of the body. However, there are other variations, such as Mosaic Down syndrome, where the extra chromosome is present in only some cells. Additionally, Translocation Down syndrome occurs when part of chromosome 21 attaches to another chromosome.