Patau syndrome is associated with severe mental retardation, small eyes that may exhibit a split in the iris (coloboma), a cleft lip and/or palate, weak muscle tone (hypotonia), an increased risk of heart defects, skeletal abnormalities, and other medical problems. Affected individuals rarely live past infancy because of the life threatening medical problems associated with this condition. Patau syndrome affects approximately 1 in 10,000 live births. The risk of having a child with Patau syndrome increases as a woman gets older.
People with Patau syndrome have additional DNA from chromosome 13 in some or all of their cells. The extra material disrupts the normal course of development, causing the characteristic features of Patau syndrome.
Most cases of Patau 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 of chromosome 13, resulting in a mixed population of cells with a differing number of chromosomes, such cases are called Mosaic Patau syndrome.
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 nondisjunction can result in reproductive cells with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 13. 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. As a result, some of the body's cells have the usual two copies of chromosome 13, and other cells have three copies of the chromosome.
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.
This article incorporates public domain text from The U.S. National Library of Medicine