Heritability of dental and skeletal cephalometric variables in monozygous and dizygous Iranian twins

Background and purpose: To assess the influence of genetic and environmental factors on craniofacial morphology in an Iranian study sample. Setting and sample population: The department of orthodontics at Islamic Azad Medical University and Iranian Twin Centre. Fifty pairs of twins were selected (25 monozygotic, MZs and 25 dizygotic, DZs). The mean age of subjects in MZ and DZ groups were 16.2 (13.4-19.8) and 16.6 (13.8-20.1) years, respectively. Methods and materials: A cross-sectional twin study carried out using lateral cephalograms. The subjects were required to pass their pubertal growth spurts and received no previous orthodontic treatment. Thirty-three linear and angular cephalometric variables were identified and used. The heritability assessments were undertaken according to the Path Analysis model and also using the Holzinger's equation. For each cephalometric variables Pearson's intra-pair correlation coefficients were calculated for MZ and DZ twin pairs. The estimate of heritability (h(2)) and coefficient of cultural heritability (c(2)) were then calculated for cephalometric variables. Results: Overall vertical variables showed higher heritability than horizontal variables. The anterior cranial base (S-N), saddle angle (NSBa), total anterior facial height (N-Me), lower anterior facial height (ANS-Me), SNA, SNB, SNPog, Gonial angle, SN-GoGn angle and SNMaxillary plane angle showed high heritability. Heritability was low to moderate for the dento-alveolar variables. Conclusions: Vertical variables (in particular total anterior facial height, TAFH and lower anterior facial height, LAFH) showed more heritability than horizontal ones. Heritability seems to be expressed more anteriorly than posteriorly. The lower third of the face seems to be under strong genetic control. (C) 2009 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved.