Down syndrome (DS) is a genetic condition resulting from an extra chromosome (number 21) in some or all of the cells. Instead of 46 chromosomes in each cell, there are 47 in each cell. Another name for Down syndrome is trisomy 21, referring to the extra copy of chromosome 21. Down syndrome can be diagnosed by physical features in most cases, and is confirmed by a karyotype usually done on a blood sample. Individuals who have Down syndrome have unique facial features and physical features, certain medical findings (such as heart defects, gastro-intestinal problems, etc), and most have an intellectual disability. There is wide variability within individuals who have this condition.
About one in every 700 babies is born with Down syndrome. Click here to read about physical features that may be associated with Down syndrome (most children with Down syndrome have only some of these features).