People with Down syndrome have a third copy of chromosome 21, instead of the usual two copies. This genetic change causes a collection of characteristics, including intellectual disability and some common physical traits.
The APP gene that leads to the production of the beta-amyloid protein present in Alzheimer's plaques is located on chromosome 21. This means that people with Down syndrome make one and a half times the amount of APP and, as a consequence, more beta-amyloid. This appears to be the cause of the earlier appearance of the brain changes typical of Alzheimer's disease in people with Down syndrome.