A couple has two children affected with tuberous sclerosis. On detailed clinical and laboratory evaluation both parents are normal. Which one of the following explains the two affected children in this family?

Germline mosaicism, Both parents are normal. Typically, a person with only germline mosaicism will not be affected with the disorder caused by the mutation because the mutation is not in the other cells of the body, Mutation occurs during post zygotic period and is only in gonadal cell. Genetic testing using blood or tissue samples (other than germline tissue) from individuals who only have a germline mutation will be negative for the mutation. Because the mosaic germline mutation is present in the egg or sperm cell, it will also be present in all cells of the child developing from that germ cell. Eg : Osteogenesis imperfecta, Tuberous sclerosis.