Down Syndrome (DS or DNS), also known as Trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is typically associated with physical growth delays, characteristic facial features and mild to moderate intellectual disability. The parents of the affected individual are typically genetically normal. The extra chromosome occurs by chance. The possibility increases from less than 0.1% in 20-year-old mothers to 3% in those age 45.

Life expectancy is around 50 to 60 years in the developed world with proper health care.

Down syndrome is one of the most common chromosome abnormalities in humans. It occurs in about one per 1000 babies born each year. In 2013, Down syndrome was present in 8.5 million individuals and resulted in 36,000 deaths down from 43,000 deaths in 1990. It is named after John Langdon Down, the British doctor who fully described the syndrome in 1866

People with Down syndrome may have some or all of these physical characteristics: a small chin, slanted eyes, poor muscle tone, a flat nasal bridge, a single crease of the palm, and a protruding tongue due to a small mouth and relatively large tongue. These airway changes lead to obstructive sleep apnea in around half of those with Down syndrome. Other common features include: a flat and wide face, a short neck, excessive joint flexibility, extra space between big toe and second toe, abnormal patterns on the fingertips and short fingers.

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