![]() Although they do not have signs of trisomy 18, people who carry this type of balanced translocation are at an increased risk of having children with the condition. This rearrangement is called a balanced translocation because there is no extra material from chromosome 18. An unaffected person can carry a rearrangement of genetic material between chromosome 18 and another chromosome. As a result, some of the body's cells have the usual two copies of chromosome 18, and other cells have three copies of this chromosome. It occurs as a random event during cell division early in embryonic development. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 18 in each of the body's cells. ![]() For example, an egg or sperm cell may gain an extra copy of chromosome 18. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. Most cases of trisomy 18 are not inherited, but occur as random events during the formation of eggs and sperm. If the entire q arm is present in three copies, individuals may be as severely affected as if they had three full copies of chromosome 18. If only part of the q arm is present in three copies, the physical signs of partial trisomy 18 may be less severe than those typically seen in trisomy 18. People with this genetic change are said to have partial trisomy 18. Affected individuals have two copies of chromosome 18, plus the extra material from chromosome 18 attached to another chromosome. Very rarely, part of the long (q) arm of chromosome 18 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) or very early in embryonic development. ![]() The development of individuals with this form of trisomy 18 may range from normal to severely affected. The severity of mosaic trisomy 18 depends on the type and number of cells that have the extra chromosome. In these people, the condition is called mosaic trisomy 18. The extra genetic material disrupts the normal course of development, causing the characteristic features of trisomy 18.Īpproximately 5 percent of people with trisomy 18 have an extra copy of chromosome 18 in only some of the body's cells. Most cases of trisomy 18 result from having three copies of chromosome 18 in each cell in the body instead of the usual two copies.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |