Down syndrome is also (or formally) known as mongolism and is the most common chromosomal abnormality in the United States. Over 350,000 people are affected by this condition. It is caused by an extra 21st chromosome. This is also known as trisomy 21. Instead of 46 chromosomes in each cell it has been determined that an extra 21st chromosome is affected in Down syndrome, this occurs in 95% of cases. There are three copies of this chromosome. It is caused by a cell error called non-disjunction. Two other types are mosaicism and translocation. Failure to disjunction causes defective cell division and produces three copies of the 21st chromosome instead of two. Both sperm and egg do not separate during division causing the extra chromosome to replicate in each cell. As mentioned above, it is responsible for 95% of Down's. Mosaicism occurs when nondisjunction occurs in a cell division after fertilization, causing a mixture of two cell types. Some of these cells contain 46 chromosomes, others contain 47 chromosomes. This type of Down syndrome is very rare and is less affected. Mosaicism is responsible for 1-2% of Down's syndrome. The last type of Down syndrome is translocation. This occurs when part of the 21st chromosome breaks off during cell division and attaches to another chromosome. The total number of chromosomes is still 46, but any more causes some characteristics of Down syndrome. This is also very rare and is responsible for 3-4% of Down's. In the 19th century, John Langdon Down was an English doctor who published an accurate description of a person with Down syndrome. Published in 1888, he was considered the "father of the syndrome". Down syndrome looks many different than a normal person. A person with this condition may have a... paper average... older (for example 48) may have a 1 in 12 chance of having a child with Down's. If a mother is younger (30 years old) she has a 1 in 900 chance of having a child affected by this condition. There is 1 in 800 live births of children affected by the disease. To detect Down syndrome there are two types of procedures for a pregnant woman. A screening test, which measures the amount of substances in the blood. This only has 60% accuracy. Another type of detection is diagnostic testing. This could cause a miscarriage due to tissue being taken from the umbilical cord. By examining children's chromosomes, this test is 98 to 99 percent accurate. There is no cure for this disease, however there are medications that help relieve symptoms a person may have or encounter. There are many other factors that determine Down syndrome and research. The NDSS, National Down Syndrome Society, sent out some brochures and you can view them.