(CNN)Here's a look at Down syndrome, a genetic condition that affects cognitive ability, causing mild to severe learning disabilities and distinctive facial characteristics. There are three types of Down syndrome: translocation, mosaicism and trisomy 21. Trisomy 21 is the most common and contributes to 95% of cases.
English physician John Langdon Down first characterized it in 1866.
In the United States, about 6,000 children are born with Down syndrome annually.
Down syndrome, without an 's, is the preferred spelling.