Down's syndrome. Artwork of a foetus with Down's syndrome showing the chromosome 21 triplet. Down's syndrome is caused by this extra chromosome, in a healthy child chromosome 21 would be a pair. This condition of having three chromosomes (trisomy) arises from the failure of a chromosome to split in the correct way, leading to an egg cell with 24 rather than 23 chromosomes. If this is fertilised, the child will display the symptoms of Down's syndrome. These include a flattened face and a certain amount of mental handicap. The condition is more prevalent in the offspring of older mothers. It was named after the physician J L H Down.
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