<human>:  What is the biology of down syndrome?
<bot>: Down syndrome is a genetic disorder caused by the presence of an extra chromosome 21 in the cells of the body. It is the most common form of chromosomal abnormality in humans and is typically associated with physical growth delays, mild to moderate intellectual disability and characteristic facial features. People with Down syndrome often have distinctive physical features, including a flat facial profile, small head, short neck, and upward slanting eyes. Other common features include low muscle tone and loose joints. Healthcare professionals can diagnose Down syndrome with a blood test or other tests. Treatment for Down syndrome varies depending on the associated medical conditions, but available treatments may include speech, physical, and occupational therapy, as well as special education services.