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History of Down Syndrome Langdon Down In 1866, a physician named John Langdon Down first described a set of children with common features who were distinct from other children with developmental delays. Dr. Down was superintendent of a facility for children with developmental delays in Surrey, England when he made this distinction. Jerome Lejune In 1959, a French physician named Jerome Lejune discovered the genetic cause of Down Syndrome, observing 47 chromosomes per cell. Cause of Down Syndrome There are three types of chromosomal abnormalities that cause Down Syndrome. Normal Disjunction In every cell of a normal person's body, there are 46 chromosomes, 23 pairs. They receive half of these chromosomes from their father, half from their mother. When the sperm and egg each divide in two, in preparation for fertilization, the chromosome pairs are also supposed to divide, one side going to each cell. This is called disjunction. Standard Trisomy 21 The first type of chromosomal abnormality that causes Down Syndrome happens when either the sperm or the egg divides wrongly. All the chromosomes except one divide correctly. In Down Syndrome, that one is the 21st chromosome. The entire 21st pair goes over to one side. After conception, there are then three total copies of the 21st chromosome, instead of the two in normal persons. As the child develops, the extra chromosome is replicated in every cell of the body. The medical term for this is Trisomy 21, or nondisjunction. It accounts for about 95% of the causes of Down Syndrome. Translocation Trisomy 21 In the second type of chromosomal abnormality that causes Down Syndrome an extra part of chromosome 21 is attached or stuck onto another chromosome. There are still only 46 chromosomes, but the extra part of number 21 is producing extra genetic products, resulting in Down Syndrome. This form is usually inherited from a parent. Sometimes a person has only one normal 21st chromosome besides the translocated one. Such a person does not have Down Syndrome. However, if one of their children has two normal 21st chromosomes, besides the extra, that extra chromosome part is producing extra genetic products, and results in Down Syndrome. This type is called translocation trisomy 21. It accounts for about 3-4% of the causes of Down Syndrome. Mosaic Trisomy 21 In the third type of chromosomal abnormality, some of the cells have a trisomy, with 47 total chromosomes, and other cells will have the normal 46 chromosomes. There have been proposed two ways that this might happen. One of the ways is that when the child was conceived, the very first cell division went wrong, the same as in standard trisomy 21. However, a little farther down the line, there was yet another error, resulting in the normal 46 chromosomes for that line of cells. If the second error occurred when there were only 2 cells, 1/2 of the chromosomes would have 46 chromosomes, and one half would have 47. If the error occurred when there were 4 cells, 1/4 of the cells would have 46 chromosomes, and 3/4 would have 47. Another of the ways suggested is that the first cell division is normal. However, nondisjunction of the 21st chromosome occurred later on. If it occurred when there were only 2 cells, 1/2 of the cells would have 47 chromosomes. If it occurred when there were 4 cells, 1/4 of the cells would have the trisomy. This type, no matter how it happened, is called mosaic trisomy 21. It accounts for about 1-2% of the causes of Down Syndrome. Since only part of the cells have a trisomy, there is not as much of the extra genetic products being produced. Therefore, depending on the percentage of trisomic cells, this child might not have as many problems as someone with standard Trisomy 21, who has all trisomic cells. Conclusion Regardless of the type of trisomy, all these people have an extra chromosome, meaning that there are extra genes producing extra genetic products. Those extra genetic products produce what we know as Down Syndrome. Trisomy 21 occurs once in every 800 to 1000 births in the United States.

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