Genetic variability of crown structure in the molar

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We compared the first molar sizes between opposite-sexed (OS) and same-sexed (SS) twins to analyze the environmental contributions to tooth size variation. In the maxilla, the mean values of the females were tended to be large in OS than in SS, but the mean values of males were larger in SS than in OS. In the mandible, the sexual differences in the tooth size showed the same pattern in both SS and OS twins. Dempsey, et al. (1999) reported that OS females had larger teeth than SS twins. They supposed that diffusion of sex hormones from male to female co-twins in utero might account for the increased tooth size in OS females. Their hypothesis was accord with the maxillary molars of our results, but our results of the mandibular molars were not accord. Our results indicated that the genetic effect was more affected in the mandibular molars than the maxilla.The cusp areas were measured on standardized photographs of the occlusal surface. Structural equation modeling was performed. There was no evidence of heterogeneity in variances of the cusp area between sexes. Genetic modeling indicated that a model incorporating additive genetic and unique environmental variation (AE model) was adequate for most of the cusp areas and crown areas. In some of these instances, a model incorporating common and unique environmental variance (CE model) provided an adequate fit. Estimates of heritability ranged from 47 to 85%. Overall crown area and crown unite areas showed higher heritability estimates than each cusp area. The paracone area showed the lowest heritability estimate in the measurements. The paracone is the first cusp in both ontogeny and phylogeny, and the most stable cusp of molars in primates. This result indicated that a stable cusp has been controlled by almost common genotypes throughout the human evolution processed.