There are three types of chromosomal patterns that result in Down syndrome. They are: trisomy 21, translocation and mosaicism. A blood test is required for chromosome analysis and the type of pattern would be determined at that time. Ninety-five per cent of people with Down syndrome have trisomy 21, which means there is an extra #21 chromosome in each cell.
Normally, each egg and sperm cell contains 23 chromosomes. The union of these creates 23 pairs, or 46 in total. Sometimes, an accident occurs when an egg or sperm cell is forming, causing it to have an extra chromosome number 21. When this cell contributes the extra chromosome 21 to the embryo, Down syndrome results. The features of Down syndrome result from having this extra chromosome 21 in each of the body's cells. This is called trisomy 21 because of the presence of three number 21 chromosomes.
Occasionally, the extra chromosome 21 is attached to another chromosome in the egg or sperm; this may result in what is called translocation Down syndrome . This is the only form of Down syndrome that can be inherited from a parent, who has a rearrangement of chromosome 21 called a balanced translocation that does not affect his or her health. Approximately 2 to 3% of people with Down syndrome have a translation pattern.
Rarely, a form of Down syndrome called mosaic Down syndrome may occur when an accident in cell division occurs after fertilization. Affected individuals have some cells with an extra chromosome 21 and others with the normal number. Approximately 2% to 3 % of people with Down syndrome have mosaicism.