Heritability of facial soft tissue growth in mono‐ and dizygotic twins at 12 and 17 years of age: A retrospective cohort study

Abstract

Objective The purpose of this investigation of untreated monozygotic and dizygotic twins was to identify the genetic and environmental component to the facial soft tissue growth. Settings and Sample Population The sample consisted of 52 untreated monozygotic twins (36 male, 16 female) and 46 untreated dizygotic twins (23 male, 23 female) from the Forsyth Moorrees Twin Study (1959-1975). Materials and Methods Lateral cephalograms were taken at 12 and 17 years of age and traced to analyze facial convexity, nasolabial angle, upper and lower lip thickness, upper and lower lip profile, and nose prominence. The genetic and environmental components of variance were analyzed with structural equation modeling for multilevel mixed-effects. Results At 12 years of age strong additive genetic influence was seen for facial convexity (70%), upper lip profile (66%) and nose prominence (65%), whereas strong dominant genetic components were found for upper lip thickness (56%). Nevertheless, under unique environment influence were nasolabial angle (58%), lower lip profile (51%) and lower lip thickness (64%). At 17 years of age, only upper lip thickness (55%) and nose prominence (84%) were under strong additive genetic control, while the rest of the variables were under strong dominant genetic control. The only exception was lower lip thickness (61%), which is still influenced by unique environment. Conclusion Although monzygotic/dizygotic twins share at least part of their genome, a both times either additive, dominant or environmental components were found. Nevertheless, at 17 years of age most of the variables are either under additive or dominant genetic influence.