Large congenital melanocytic nevi may reflect paradominant inheritance implying allelic loss.

Large congenital melanocytic nevi have so far always been considered to occur sporadically, and until now little has been written about a possible role of heredity as a cause of this disorder. We reviewed familial cases of large congenital melanocytic nevi as reported in the literature and found 14 case reports with a family history of congenital nevi. We propose the concept of paradominant inheritance as a possible genetic explanation. The concept would imply that heterozygous individuals are phenotypically normal which is why the mutation would be transmitted unperceived through many generations. The trait would become manifest only when loss of heterozygosity occurred at an early developmental stage, giving rise to a patchy area of homozygous or hemizygous cells. This would explain why the lesions of large congenital melanocytic nevi are always arranged in a mosaic pattern; why they occur virtually always sporadically; and why the exceptional cases of a familial aggregation of this trait do not show any consistent Mendelian pattern.