A peculiar observation published in 1971 by Chernosky et al. is revisited in this paper. An African American mother and three of her four children (two girls and one boy) had hyperpigmented skin areas arranged along Blaschko’s lines. According to the current knowledge of formal genetics, a mendelian mode of transmission can be excluded, and paradominant inheritance is likewise highly unlikely. The unusual family constellation of pigmentary mosaicism can be best explained as an example of monoallelic autosomal expression, a concept that is now well established in the genetics of plants and animals but so far unexplored in human skin disorders. Either the paternal or the maternal allele is randomly inactivated, therefore this mechanism can be taken as an autosomal counterpart of X-chromosome inactivation. Recent studies suggest that 5–10% of autosomal human genes are monoallelically expressed. This theory opens a new field of research in dermatology. Clinicians should consider this new aetiological concept when observing cases of hereditary cutaneous mosaicism that cannot be explained by X-linkage.