“Down syndrome is the most common and readily identifiable chromosomal condition associated with intellectual disabilities.It is caused by a chromosomal abnormality: for some unknown reason, an accident in cell development results in 47 instead of the usual 46 chromosomes. This extra chromosome changes the orderly development of the body and brain. In most cases, the diagnosis of Down syndrome is made according to results from a chromosome test administered shortly after birth.
Just as in the normal population, there is a wide variation in mental abilities, behavior, and developmental progress in individuals with Down syndrome. Their level of intellectual disability may range from mild to severe, with the majority functioning in the mild to moderate range.”
It’s very helpful to read more about down syndrome. Following are links to additional information: