What is characterized by an extra copy of chromosome 21?

Down syndrome is characterized by an extra copy of chromosome 21, which results from a genetic error during the formation of reproductive cells or in early fetal development. This condition, also known as Trisomy 21, leads to distinctive physical features, developmental delays, and potential health issues due to the additional genetic material impacting a person’s bodily systems.

Understanding Down syndrome involves recognizing how the presence of an extra chromosome can affect an individual's development and health, making it distinct from the other listed syndromes. Fetal Alcohol Syndrome results from maternal alcohol consumption during pregnancy and is not related to chromosomal changes. Fragile X syndrome is a genetic condition caused by a mutation in the FMR1 gene on the X chromosome, and Twins syndrome does not refer to a specific genetic condition but rather describes the phenomenon of multiple pregnancies.