1. Trisomy 21: The Fetus takes a third copy of chromosome 21 instead of two copies, which is repeated three times instead of twice. The number of chromosomes increases to 47 instead of the normal 46 in each cell and this type represents 95% of Down’s Syndrome cases.
2. Dislocation: Where chromosome number 21 becomes dislocated and sticks to another chromosome, usually number 14 but the number of chromosomes remains the normal 46. This type represents about 4% of Down’s Syndrome cases - half of which, are hereditary and half sporadic.
3. Mosaic Down’s Syndrome (mosaicism): In this type, there are two types of cells, some contain the normal 46 chromosomes and others contain 47 chromosomes. This type represents about 1% of Down’s Syndrome cases.