Wikipedia states that, "Down syndrome, Down's syndrome, or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British doctor who described the syndrome in 1866. The disorder was identified as a chromosome 21 trisomy by Jérôme Lejeune in 1959. The condition is characterized by a combination of major and minor differences in structure. Often Down syndrome is associated with some impairment of cognitive ability and physical growth as well as facial appearance. Down syndrome can be identified during pregnancy or at birth." People with Down Syndrome usually have lower than average cognitive ability and may have learning disablilities as well. It is estimated that 1 per 800 to 1,000 births result in babies with Down Syndrome. Expectant mother's over the age of 35 are at greater risk of having a baby with Down Syndrome. One out of every nineteen babies from mothers over the age 45 have Down Syndrome. Many standard prenatal screens can discover Down syndrome. Ask your doctor about these screenings if you are at risk.
Some physical characteristics of individuals with Down Syndrome include, muscle hypotonia (poor muscle tone), a protruding tongue (due to small oral cavity, and an enlarged tongue near the tonsils), a short neck, oblique eye fissures with epicanthic skin folds on the inner corner of the eyes, excessive joint laxity including atlanto-axial instability, and congenital heart defects. These are not all of the characteristics of Down Syndrome and people with this disorder may have just some or all of these.