Genetic variability of crown structure in the molar

我们比较了异性（OS）和同性（SS）双胞胎的第一磨牙大小，以分析环境对牙齿大小变化的影响。在上颌骨中，雌性的平均值在正常组中比正常组大，而雄性的平均值在正常组中比正常组大。在下颌骨，牙齿大小的性别差异在SS和OS双胞胎中表现出相同的模式。Dempsey等（1999）报道OS雌性比SS双胞胎的牙齿更大。他们认为性激素在子宫内从男性扩散到女性双胞胎可能解释了OS女性牙齿尺寸增加的原因。他们的假设与我们的上颌磨牙结果一致，但我们的下颌磨牙结果不一致。结果表明，遗传效应对下颌磨牙的影响大于对上颌骨的影响。在咬合面的标准化照片上测量牙尖面积。进行了结构方程建模。没有证据表明两性间的尖面积差异具有异质性。遗传模型表明，加性遗传和独特环境变异模型（AE模型）适用于绝大多数尖区和树冠区。在其中一些情况下，一个结合了常见和独特环境差异的模型（CE模型）提供了足够的拟合。估计遗传率从47%到85%不等。总体冠面积和冠单位面积的遗传力估计高于每个尖面积。副锥体区域的遗传力估计值最低。副尖牙是个体发育和系统发育的第一个尖牙，也是灵长类动物臼齿最稳定的尖牙。这一结果表明，在整个人类进化过程中，一个稳定的尖点被几乎常见的基因型所控制。

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.