The clinical situation when a person has two populations of cells in the gonads (testes or ovaries), one population of cells containing the usual genetic complement whilst the other contains a DNA mutation or chromosome anomaly.
The genetic change is confined solely to the germline (the cells which produce the gametes) of the parent so that the other cells in the person's body have the usual genetic complement.
If a sperm or an egg produced from the cells in the parent's gonads containing the DNA mutation or chromosome anomaly is used to form a fetus, the child will have the genetic condition. Although the parent is healthy, he or she could have another child with the same genetic condition if the child is formed from a sperm or egg from the patch of cells in the gonad which contains the genetic change. A child would not have the condition if formed from the cells in the gonad with the usual genetic pattern.
Gonadal/germline mosaicism is a likely explanation of the rare situations where a person without a dominant condition can have two children with the same autosomal dominant condition