Trisomy 21 Symptoms Trisomy 21 is a disorder caused by three chromosomes 21, also known as Down syndrome. Common symptoms include mental retardation, deformed facial features, heart and digestive system abnormalities, and certain growth and development disorders. One of the most prominent symptoms is mental retardation. Patients usually do things more slowly than their peers and have difficulty mastering basic skills such as language skills, mathematical understanding, and memory. In terms of facial features, people with Down syndrome usually have small heads, flat faces, and slanted faces.