What is trisomy 13?

Trisomy 13, also called Patau syndrome, is a chromosomal condition that is associated with severe mental retardation and certain physical abnormalities. These abnormalities include small eyes that may exhibit a split in the iris (coloboma), an opening in the roof of the mouth (a cleft palate) and/or a cleft lip, weak muscle tone (hypotonia), skeletal abnormalities, an increased risk of heart defects, and other medical problems. Affected individuals rarely live past infancy because of the life-threatening medical problems associated with this condition.

How common is trisomy 13?

Trisomy 13 affects approximately 1 in 10,000 newborns. The risk of having a child with trisomy 13 increases as a woman gets older.

What are the genetic changes related to trisomy 13?

Trisomy 13 is a chromosomal condition related to chromosome 13.

People with trisomy 13 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 trisomy 13.

Most cases of trisomy 13 result when 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 sometimes called mosaic trisomy 13.

Trisomy 13 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 those typically seen in trisomy 13.

Can trisomy 13 be inherited?

Most cases of trisomy 13 are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction results 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 cell of the body.

Mosaic trisomy 13 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.

Translocation trisomy 13 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 trisomy 13, people who carry this type of balanced translocation are at an increased risk of having children with the condition.

Source: National Institutes of Health

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