This extra chromosome can be present in three different ways. Standard Trisomy 21, Translocation, and Mosaicism. Standard Trisomy 21 occurs when the extra chromosome comes from either the egg or the sperm. There is an extra identical copy of the chromosome in each cell. Ninety to ninety-five percent of all cases of Down syndrome are this type.
Translocation occurs when the extra chromosome attaches itself to another chromosome.
It does not join the other two number twenty-one chromosomes. The extra genetic material in the cell is the same, leading to the same clinical picture as standard Trisomy 21. This accounts for three to five percent of Down syndrome cases. Mosaicism occurs when some cells, but not all the cells in the body, contain the extra twenty-first chromosome. As a result, these individuals may exhibit none, some, or all of the characteristics of Down syndrome.