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Trisomy 21 Down syndrome happens when an extra copy of chromosome 21 is present in all cells of the body.
A person with trisomy 21 will have three copies of chromosome 21 in every cell of the body. A typical person has just two copies of chromosome 21. The picture below shows what the chromosomes look like in one cell of a person with trisomy 21.
Trisomy 21 Down syndrome happens by chance. It is not passed down by parents.
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