Down Syndrome (cont.)
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What is Down Syndrome (the Genetics)
Down syndrome is a genetic disorder caused by the presence of an extra chromosome 21. Chromosomes contain the genes that carry all the information necessary to properly develop and maintain our bodies.
Human cells normally contain 46 chromosomes that can be arranged in 23 pairs. One set of 23 chromosomes comes from the mother (egg cell or ovum) and the other half of the 23 pairs comes from the father (sperm cell). In most cases of Down syndrome, the person inherits two chromosomes 21 from the mother (instead of one) and one chromosome 21 from the father, resulting in three copies of chromosome 21 instead of two (hence Down syndrome is also known as trisomy 21). In Down syndrome, the additional copy of chromosome 21 results in the increased expression of genes located on this chromosome. It is believed that the activity of these extra genes leads to many of the features that characterize Down syndrome.
Down Syndrome - Trisomy 21
Individuals who inherit an entire extra chromosome 21 make up approximately 95% of Down syndrome cases. As mentioned above, this usually happens when the individual inherits two copies of chromosome 21 (instead of one) from the mother's egg during fertilization. In rare cases, the individual inherits the extra chromosome 21 through the father's sperm. In either case, it leads to a fertilized egg with three copies of chromosome 21 instead of two. To date, it is not known what causes the inheritance of the extra chromosome 21. The only known risk factor for Down syndrome is the mother's age at conception; the older the mother, the higher the risk of conceiving a baby with Down syndrome.
Robertsonian Translocation and Partial/Segmental Trisomy
In some people, parts of chromosome 21 fuse with another chromosome (usually chromosome 14). This is called a Robertsonian translocation. The person has a normal set of chromosomes, but one chromosome contains extra genes from chromosome 21. When a person with a Robertsonian translocation has a child, the extra genetic material from chromosome 21 is inherited and the child will have Down syndrome. Robertsonian translocations occur in 3% to 4% of Down syndrome cases.
In extremely rare cases, very small pieces of chromosome 21 are incorporated into other chromosomes. This is known as partial or segmental trisomy 21.
Mosaic Down Syndrome
About 2% to 4% of Down syndrome cases are mosaic. In mosaic Down syndrome, some cells in the body have three copies of chromosome 21 and the rest of the cells are unaffected. For example, a person might have skin cells with trisomy 21, while all other cell types are normal. Mosaic Down syndrome can sometimes go undetected, because a person with mosaic Down syndrome does not necessarily have all the characteristic physical features and often is less cognitively impaired than a person with trisomy 21. A person with mosaic Down syndrome may also be misdiagnosed as having trisomy 21.
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