Down syndrome a congenital condition characterized by a distinctive pattern of physical characteristics including a flattened skull, pronounced folds of skin in the inner corners of the eyes, large tongue, and short stature, and by some degree of limitation of intellectual ability and social and practical skills. It usually arises from a defect involving chromosome 21, usually an extra copy (trisomy-21).

The parents of the affected individual are usually genetically normal. The probability increases from less than 0.1% in 20-year-old mothers to 3% in those of age 45. The extra chromosome is believed to occur by chance, with no known behavioral activity or environmental factor that changes the probability. Down syndrome can be identified during pregnancy by prenatal screening followed by diagnostic testing or after birth by direct observation and genetic testing. Since the introduction of screening, Down syndrome pregnancies are often aborted.

There is no cure for Down syndrome. Education and proper care have been shown to improve quality of life. Some children with Down syndrome are educated in typical school classes, while others require more specialized education