Trisomy 18 is a chromosomal disorder characterized by several physical and intellectual features. Babies with trisomy 18 may show various signs and symptoms, including low birth weight, a small head, clenched hands, heart defects, and kidney problems. Cases can vary greatly from one individual to another.
The worldwide incidence of trisomy 18 is estimated at 1 in 5000 live births, and the majority of babies born with trisomy 18 are female. Complications arising from the condition are often fatal, with most babies dying within a few weeks of being born. Treatment and management depend on the specifics of each case.
The Genetics of Trisomy 18
Typically, humans have two copies of chromosome 18 in each of their somatic (body) cells. In trisomy 18, there are three copies present, which is referred to as a full trisomy. This extra chromosome 18 material affects health and development, resulting in the signs and symptoms of trisomy 18.
Rarely, trisomy 18 can be present in some body cells but not others (mosaicism). Mosaic trisomy 18 is sometimes less severe than a full trisomy. Very rarely, only a piece of a chromosome 18 is present in cells. This is called a partial trisomy, and can happen when a portion of chromosome 18 attaches to another chromosome (translocation).
Is Trisomy 18 Inherited?
Trisomy 18 is usually not inherited. The chromosome imbalance is typically the result of an error in cell division. Such errors are random occurrences that take place either early in the embryo’s development, or before conception, when eggs and sperm are being made. The translocation type of trisomy 18 can be inherited.
Genetic Testing and Trisomy 18
A chromosome analysis can detect abnormalities such as an extra chromosome, or other problems with chromosome number and structure. This can be carried out prenatally as well as after birth. Genetic counseling may be available to individuals and families who would like support and information on trisomy 18.
Visit the Support Organization for Trisomy 18, 13, and Related Disorders (SOFT) and the Trisomy 18 Support Foundation websites for more information on the features, testing and treatment involved in trisomy 18. For more details on chromosomes and human health, read Introduction to Chromosomes in Human Cells and Introduction to Chromosome Abnormalities.
Sources
“Trisomy 18”, published: 7th June 2010, Genetics Home Reference website, accessed: 12th June 2010, no author specified.
“Trisomy 18”, last updated: 11th August 2009, MedlinePlus (National Library of Medicine) website, accessed: 20th June 2010, author: Chambers, Diana.
“Trisomy 18 Facts”, Support Organization for Trisomy 18, 13 and Related Disorders (SOFT) website, accessed: 12th June 2010, Carey, John.
“What Is Trisomy 18”, Trisomy 18 Foundation website, accessed: 12th June 2010, no author specified.
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