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Patau syndrome, also known as trisomy 13, is a chromosomal abnormality, a syndrome in which a patient has an additional chromosome 13 due to a non-disjunction of chromosomes during meiosis. Some are caused by Robertsonian translocations. The extra chromosome 13 disrupts the normal course of development, causing the characteristic features of Patau syndrome. Like all non-disjunction diseases (Down syndrome, Edwards syndrome, etc.), the risk of disease in the offspring increases with maternal age at pregnancy, with about 31 years being the average.[1] Patau syndrome affects approximately 1 in 12,000 live births.
Most cases of Patau’s syndrome result from trisomy 13, which means each cell in the body has three copies of chromosome 13 instead of the usual two copies. A small percentage of cases occur when only some of the body’s cells have an extra copy, resulting in a mixed population of cells with a differing number of chromosomes; such cases are called mosaic Patau.
Patau syndrome can also occur when part of chromosome 13 becomes attached to another chromosome (translocated) before or at conception. Affected people have two copies of chromosome 13, plus extra material from chromosome 13 attached to another chromosome. With a translocation, the person has a partial trisomy for chromosome 13 and often the physical signs of the syndrome differ from the typical Patau syndrome.
Most cases of Patau syndrome are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called non-disjunction can result in reproductive cells with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of the chromosome. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 13 in each of the body’s cells. Mosaic Patau syndrome is also not inherited. It occurs as a random error during cell division early in fetal development.
Patau syndrome due to a translocation can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 13 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 13. Although they do not have signs of Patau syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition.
Of those embryos that do survive to gestation and subsequent birth, common abnormalites include:
Unless one of the parents is a carrier of a translocation the chances of a couple having another trisomy 13 affected child is less than 1% (less than that of Down Syndrome).
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