Down syndrome is the most common birth defect in the United States, occurring in approximately 1 in 800 newborns. This birth defect has no treatment to cure the disease. This disease was discovered in 1866 and named after the doctor who discovered it, John Langdon Down. There are three different types of Down syndrome, the most common being trisomy 21. Trisomy-21 is an extra copy of the 21st chromosome. Translocation is the second most common form of Down syndrome. Part or all of the 21st chromosome is attached to another chromosome. The latest form of Down syndrome is called Mosaic Down syndrome. In this form some cells have an extra chromosome 21 while some cells have two normal ones. Sometimes problems occur before fertilization, as in the case of trisomy 21 and translocation, when an egg or sperm divides incorrectly leaving it with an extra copy of chromosome 21, or a piece of chromosome 21 breaks off and attaches to another chromosome. Unlike trisomy 21 and translocation Down syndrome, mosaic Down syndrome occurs after fertilization. The chance of having a child with Down syndrome increases as the mother ages. According to the American Pregnancy Association the risk for mothers in their 20s is 1:1600, 25 is 1:1300, 30 is 1:1000, 35 is 1:365, 40 is 1:90 and 45 is 1:30 (APA ). Down syndrome is a lifelong disorder, but many affected individuals survive into adulthood. Down syndrome affects nearly 350,000 people in the United States. Individuals with Down syndrome have altered physical appearances that include: flattened faces, upward-slanting almond-shaped eyes, short necks, small ears, small hands and feet, white spots on the iris, short stature during childhood and ... .. middle of paper ......ask about some birth defects affecting the brain and spine. Another way to prevent Down syndrome is genetic engineering. With IVF, doctors can test a woman's eggs to determine whether they carry certain genetic diseases. After the determination, doctors use only embryos that do not carry the affected genes and then implant the embryos into the woman. This important medical discovery can prevent incurable diseases, such as Down syndrome. Down syndrome is a disease that affects the 21st chromosome by having an extra copy, located somewhere else or just some cells containing the extra chromosome while other cells do not. The disease is incurable but not fatal. New scientific findings may help prevent the disease through in vitro fertilization and by ensuring the disease is not present in embryos before implantation.