Paradominance and Didymosis

Paradominance, a recently described mode of inheritance, has been postulated by Happle to explain a familial aggregation of some mosaic disorders that imitates an autosomal dominant transmission. Additional mosaic disorders in the form of cutis marmorata telangiectatica congenita and large congenital melanocytic nevus are proposed and documented as possible paradominant traits. An analysis of familial aggregations conclude that no characteristic pedigree pattern can be ascribed to paradominant inheritance. The familial constellation may resemble autosomal dominant inheritance but may also mimick autosomal recessive inheritance. Didymosis is defined as paired patches of mutant tissue that differ genetically from each other and from the heterozygous background tissue. Both components of didymosis can be separately transmitted in the form of a paradominant trait; these separately transmitted components would only become manifest when at an early developmental stage, a postzygotic mutation occurred, giving rise to loss of heterozygosity resulting in a homozygous or hemizygous cell population that survives in close proximity with the phenotypically normal heterozygous cell population reflecting the action of a paradominant gene.