Paradominance

Among the mosaic phenotypes, which are caused by lethal mutations and therefore occur predominantly sporadically, there are also those which exceptionally occur familially clustered, for example nevus sebaceus or nevi flammei including Sturge-Weber-Klippel-Trénaunay syndrome.

The concept of paradominance has been postulated as an explanation. Heterozygous gene carriers are phenotypically healthy, and therefore the mutation can be passed on unnoticed for many generations. Only when loss of heterozygosity occurs in a somatic cell at an early stage of embryogenesis does the trait manifest as mosaicism.