There are three ways that Down syndrome is caused. The most common cause of Down syndrome is trisomy. Trisomy is when the child receives two chromosomes from the mother and one from the father. This makes it so there are three chromosomes of chromosome #21. Another way Down syndrome is caused is when new cells are made. Sometimes even when the parent cells are normal chromosome 21 can be deformed when cells reproduce. This makes it so some cells have 47 chromosomes and others have 46. This is called a mosaic disorder. Mosaic means that they have a third chromosome from the replication of cells. The third way Down syndrome can be caused is called translocation. This happens when a normal chromosome breaks into two pieces. This results in three chromosomes.