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Down's syndrome is named after John Langdon Down, the British physician who first described the syndrome. A normally developed human being has 46 chromosomes in 23 pairs, of which one pair is sex chromosomes, determining the gender. The other 22 pairs are identical for males and females. Those born with Down's syndrome have one extra chromosome, i.e. a total of 47 chromosomes. Because it is the presence of an extra 21st chromosome that causes Down's, the disorder is also known as 'trisomy 21'.