Influence of genetic and environmental factors on transverse growth

The aim of the study was to determine the genetic and environmental effects on transverse growth of craniofacial structures, within and between identical and fraternal twins. The sample consisted of 142 children in total, divided into 29 pairs of monozygotic twins, 42 pairs of dizygotic twins, and 1 set of dizygotic triplets. Postero-anterior cephalometric radiographs were taken at the ages of 9, 12, and 15 years. Intercanine width, maxillary width, mandibular width, nasal width, and facial width variables were measured. The genetic and environmental components of variance were analyzed with structural equation modeling for multilevel mixed effects. Intercanine width was initially mainly characterized by a moderate genetic component at 9 years (53%), with environmental influence increasing at age 12 (36%) and peaking at 15 years (84%). Maxillary width was under strong genetic influence at 9 years (70%), with genetic influence remaining strong up to 15 years (73%). Mandibular width was under additive genetic influence at 9 years (76%), with dominant genetic influence remaining high at 15 years (81%). Nasal width was under strong additive genetic influence at 9 years (69%) but switched to increased environmental influence at 15 years (59%). Finally, facial width had a moderate genetic influence at 9 years (66%), which increased at 15 years (90%). This study included patients of European descent, which may limit the generalizability of the findings to other ethnic groups. Although monozygotic and dizygotic twins share at least part of their genetic material, environmental factors accounted for about 10%-84% of variability at various ages, with intercanine width being most affected.