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Trisomy is a genetic condition in which cells have an extra chromosome. Normally, each cell carries 23 pairs of chromosomes. At conception, when cells begin to divide, an extra chromosome may attach to a pair of chromosomes. This creates cells with 47 chromosomes rather than 46. The extra chromosome is usually expressed with distinctive physical and mental characteristics.
Full trisomies occur when every cell in the body has the extra chromosome. In mosaic trisomies, only some cells contain the extra chromosome. People with partial trisomies have just part of the extra chromosome in their cells. Both mosaic and partial trisomies may be hereditary, while full trisomies are not. Trisomies are not caused by environmental factors or parental influence.
Trisomies are numbered according to the location of the extra chromosome. There are 23 chromosome locations. Trisomy 21, or Down syndrome, means that there are three chromosomes in the 21st location. As with all trisomies, the risk for Down syndrome increases with the age of the mother at the time of birth. A 45-year-old woman has a 1 in 30 chance of giving birth to a Down syndrome child.
Edwards syndrome, or Trisomy 18, is the second most common trisomy. It occurs in 1 out of 3,000 live births. Edwards syndrome is usually fatal, with less than 10 percent of babies born with this syndrome living more than one year. The syndrome causes heart defects, kidney abnormalities, and severe developmental delays.
Patau syndrome is a result of an extra chromosome in the 13th location, or Trisomy 13. Like Edwards syndrome, less than 10 percent of babies born with Patau syndrome live more than one year. This syndrome occurs in 1 out of 10,000 live births. It can cause structural problems with the brain, severe mental retardation, and heart defects.
Trisomies can occur on any of the other 20 chromosomes, but they rarely result in live births. There are isolated cases of children with rarer trisomies surviving for several years. Trisomy 16 occurs in one percent of all pregnancies, but the pregnancies usually result in miscarriages during the first trimester.
A diagnosis usually occurs during pregnancy. An ultrasound exam may detect problems with the baby’s heart or kidneys, and an amniocentesis procedure is then used to confirm the diagnosis. In the case of live births, a tentative diagnosis can be made by a physician based on distinctive physical characteristics. A chromosomal analysis called a kerotype is used to isolate the chromosomes and confirm the diagnosis.
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