Which genetic condition is caused by an extra 21st chromosome?

The condition that results from an extra 21st chromosome is known as Down syndrome. This genetic abnormality occurs when an individual has three copies of chromosome 21 instead of the typical two. This additional genetic material affects physical growth, cognitive ability, and is associated with certain health issues.

The presence of an extra chromosome during cell division leads to changes in the development of the body and brain, which can result in distinct physical features and varying degrees of intellectual disability. Down syndrome is one of the most common chromosomal conditions and is a major focus in pediatric genetics and developmental studies due to its implications for child development and family dynamics.

In contrast, Turner syndrome is caused by the complete or partial absence of one of the two X chromosomes in females, Fragile X syndrome is linked to a mutation on the FMR1 gene located on the X chromosome, and Klinefelter syndrome involves the presence of one or more additional X chromosomes in males. Each of these conditions has different causes and characteristics that differentiate them from Down syndrome.