Definition

A genetic condition caused by having an extra chromosome 21 in some or all of the body’s cells. Trisomy 21 is marked by growth, developmental, and learning delays that vary from mild to severe. People with trisomy 21 often have certain distinct physical features, such as a shorter-than-normal height, a flat face with a short nose, eyes that are slanted and almond-shaped, small ears, a short neck, a tongue that tends to stick out of the mouth, small hands and feet, and a deep line that goes across the palm of the hand. People with trisomy 21 may also have muscle weakness, loose joints, heart defects, and other health problems. Having trisomy 21 increases the risk of developing Alzheimer disease at an early age, usually around the age of 50, and leukemia, especially in young children. Trisomy 21 is usually not inherited (passed from parent to child). Also called Down syndrome.