What is Down syndrome? Down syndrome is a common chromosomal abnormality caused by the presence of three copies of chromosome 21 (or part of chromosome 21). Therefore, Down syndrome is also called trisomy 21 syndrome. Because the genetic mutation for the disease occurs during sperm or egg formation, Down syndrome is not caused by intermarriage between relatives. Five Characteristics Down syndrome usually manifests itself in five main ways. Below is a detailed description of these aspects.