Last Reviewed May 29, 2009
In genetics, mosaicism is characterized by the presence of one or more cell lines in a single person.
Germline mosaicism (formerly known as gonadal mosaicism) is a form of mosaicism confined to the germ and germline cells, in which a proportion of cells have a mutation, while the rest do not. Germ cells are the sex cells (egg and sperm), while germline cells are cells from which the egg and sperm orginated.
A person with germline mosaicism will not be affected with the genetic condition caused by the mutation because the mutation is not present in the rest of the body's cells. As such, genetic testing using blood or non-germline tissue samples from people with a germline mutation will be negative.
People with germline mosaicism are at risk to have children with the genetic condition caused by the mutation. If an egg or sperm cell with the mutation is used to conceive a pregnancy, that mutation will be present in all of the cells of the developing child.
Germline mosaicism can be found in disorders with any pattern of inheritance, but it is most commonly seen in conditions with autosomal dominant or X-linked patterns of inheritance. Examples of conditions in which germline mosaicism has been documented include neurofibromatosis type 1 and Duchenne Muscular Dystrophy.
Typically, a person is unaware they have germline mosaicism until they have an affected child. The recurrence risk for a parent with germline mosaicism to have another affected child is difficult to predict. Regarding conditions with autosomal dominant inheritance, studies have demonstrated that the risk to have another affected child may be low (about 1%), moderate (about 6%), or high (about 30%), depending on the proportion of germ cells (egg and sperm) with the mutation as well as the disorder itself.