Is Trisomy 18 inherited ?
Is trisomy 18 inherited? Most cases of trisomy 18 are not inherited and occur randomly due to errors in the formation of eggs or sperm. If an egg or sperm gains an extra copy of chromosome 18 during cell division and contributes to a pregnancy, the embryo will have an extra chromosome 18 (trisomy) in each cell of the body. Mosaic trisomy 18 (when some body cells have trisomy 18 and some have a normal chromosome make-up), is also typically not inherited. Mosaic trisomy 18 is also due to an error in cell division, but the error occurs early in embryonic development. About 5% of affected people have a mosaic form of trisomy 18. Partial trisomy 18 (when only part of chromosome 18 is present in 3 copies) can be inherited. An unaffected parent can carry a rearrangement of genetic material between chromosome 18 and another chromosome. This rearrangement is called a balanced translocation because there is no extra or missing genetic material. However, a person with a balanced translocation has an increased risk with each pregnancy to have a child with trisomy 18.