Since the 19th century, and in every field of medicine, monozygotic twins have been studied to assess the involvement of genetic and environmental factors in phenotypic expression. The phenotype/genotype relationship remains the leading problem in contemporary biology. In dentofacial orthopedics, this relationship is of relevance in the three-dimensional approach to the face, in both diagnosis and treatment. The present study of two monozygotic twins presenting skeletal class III malocclusions which were genetic yet different is a clear illustration of the interaction of genotype and epigenetic factors with environmental influences. We will demonstrate that treatment can reduce phenotypic differences.