APA Dictionary of Psychology

a chromosomal disorder characterized by an extra full or partial copy of chromosome 21 and associated with physical and intellectual disabilities, developmental delays, and characteristic physical features, such as a flattened nasal bridge and slanted eyes. Brain size and weight are below average, and individuals usually have mild to severe intellectual developmental disorder, and slow, clumsy, and/or uncoordinated motor movements. Individuals with Down syndrome may have heart defects and respiratory insufficiencies or anomalies that are often corrected during infancy by surgery, as well as hearing loss, vision problems, and thryoid conditions. Lifespan is reduced compared with that in the general population, and individuals with Down syndrome sometimes show early onset of Alzheimer’s disease. Down syndrome is one of the most common physiological causes of intellectual developmental disorder. Also called Langdon Down’s disease; trisomy 21. See also autosomal trisomy of group G. [described in 1866 by John Langdon Haydon Down (1828–1896), British physician]