What is 21 Trisomy?

Question

The 21st chromosome in the human karyotype has an extra chromosome which leads to a condition called Down Syndrome.

Answer 1

Trisomy 21 is a condition where a person has three copies of chromosome 21 instead of the usual two. This extra copy changes how their body and brain develop. It can lead to things like lower muscle tone, a flatter facial profile, and learning differences. It’s one of the most common genetic conditions, and doctors can detect it before or after birth with a genetic test.

Example: If you looked at a person’s karyotype (a picture of all their chromosomes), you’d normally see two chromosomes at each pair. But in someone with Trisomy 21, you’d see three chromosomes at the 21st pair and that’s what causes Down Syndrome.

Answer 2

Trisomy 21, also known as Down syndrome, is a genetic disorder caused by the presence of an extra copy of chromosome 21. Typically, individuals have 46 chromosomes; in the case of Trisomy 21, individuals will have an extra chromosome on chromosome 21, resulting in a total of 47 chromosomes. This condition most commonly results from a cell division error called nondisjunction, which occurs when cells fail to separate properly during mitosis or meiosis, resulting in abnormal chromosome numbers. People with Down syndrome often exhibit characteristic physical features such as a flat facial profile, upward-slanting eyes, and low muscle tone, along with mild to moderate intellectual disability and developmental delays.