GENETIC COVARIANCE STRUCTURE OF INCISOR CROWN SIZE IN TWINS

Previous studies of tooth size in twins and their families have sugges ted a high degree of genetic control, although there have been difficu lties separating the various genetic and environmental effects. A gene tic analysis of variation in crown size of the permanent incisors of S outh Australian twins was carried out, with structural equation modeli ng used to determine the relative contributions of genetic and environ mental factors. Maximum mesiodistal crown dimensions of maxillary and mandibular permanent incisors were recorded from dental models of 298 pairs of twins, including 149 monozygous (MZ) and 149 dizygous (DZ) pa irs. The analysis revealed that: (i) an adequate fit required additive genetic and unique environmental components; (ii) augmenting the mode l with non-additive genetic variation did not lead to a significant im provement in fit; (iii) there was evidence of shared environmental inf luences in the upper central incisors of males; (iv) the additive gene tic component constituted a general factor loading on all eight teeth, with group factors loading on antimeric pairs of teeth; (v) unique en vironmental effects were mostly variable-specific; (vi) most factor lo adings on antimeric tooth pairs could be constrained to be equal, indi cating a symmetry of genetic and environmental influences between left and right sides; and (vii) estimated heritability of the incisor mesi odistal dimensions varied from 0.81 to 0.91.