According to Mayo Clinic, Down syndrome occurs in three variations and is caused by additional genetic material from chromosome 21. Cell division errors cause the chromosome abnormalities. The majority of Down syndrome cases, approximately 95 percent, are trisomy 21, which results from three copies of chromosome 21 instead of two copies.Continue Reading
In the rare form of mosaic Down syndrome, some cells have a superfluous copy of chromosome 21. With translocation Down syndrome, part of chromosome 21 is moved onto another chromosome.
The National Institute of Child Health and Human Development (NICHD) says that in more than 90 percent of Down syndrome cases, the additional chromosome 21 copy comes from the mother's egg. The father's sperm contributes that extra copy in about 4 percent of cases. The rest of the time, the cell division abnormality happens as the embryo grows post-fertilization.
NICHD notes that it is possible for a person to have a chromosome 21 translocation without having Down syndrome. However, that person may pass that translocation on to his or her children, causing Down syndrome. Mayo Clinic says that this is the only type of Down syndrome that can be passed from parent to child, and it comprises only about 33 percent of Down syndrome translocation cases. The chances of passing it on are higher with the mother, a 10 to 15 percent risk, than with the father, an approximately 3 percent risk.Learn more about Conditions & Diseases