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Mosaic Down syndrome happens when an extra copy of chromosome 21 is present in some, but not all, of the body’s cells.
Mosaic Down syndrome is caused by a random event shortly after the egg and sperm join together.
Some cells have two copies of chromosome 21, while other cells have three copies of chromosome 21. The diagrams below show what two different kinds of cells look like in the same person.
Mosaic Down syndrome happens by chance. It is not passed down by parents.
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