Down Syndrome

Down syndrome, named after the physician Langdon Down, is the most common genetic form of mental retardation, occurring in 1 to 1.5 of every 1,000 live births. In approximately 95 percent of cases, Down syndrome results from an extra chromosome on the twenty-first of the twenty-three pairs of human chromosomes. Exactly what causes the addition of the extra chromosomal material associated with Down syndrome is not clear, although risk factors include maternal age and possibly paternal age. People with Down syndrome usually function in the moderate range of mental retardation, with IQs generally ranging from 40 to 55 on average, though IQs can sometimes be higher or lower. People with Down syndrome experience particular deficits in certain aspects of language development, particularly expressive language, articulation, and grammar. Despite these deficits, many individuals with Down syndrome have relatively good social skills.