Torus mandibularis has frustrated several attempts to make family patterns of variation fit modifications of Mendelian models. It is suggested that the quasi-continuous model of inheritance provides a rational explanation for the diverging opinions. The model implies an underlying continuous and normally distributed variable, 'liability', with a threshold value beyond which individuals will be affected. Both genetic and environmental factors determine liability, making the system multifactorial. The incidence of variable degrees of torus was examined in two groups of patients with different stresses on the jaws: one group with bruxism and one comparison group. The transformation of incidences to group means and variances of liability was demonstrated. Muscular forces during bruxism were shown to influence liability. The relative importance of environmental and genetic components of variance could, however, not be estimated directly from the entire groups, since both were mixed with regard to the genetic predisposition. To achieve materials with uniform genotypes, all individuals without torus were omitted. The estimate of the genotypic variance (VG) was obtained by subtracting the variance of the bruxism sub-group--the environmental component associated with bruxism (VEB)--from the total phenotypic variance of the comparison sub-group (VP). The estimate of the genetic determination of torus (VG/VP) turned out to be about 30%, whereas approximately 70% of the causes seemed to be attributable to environmental influence in terms of occlusal stress. Gene effects on the morphologic level are usually pleiotropic, and it is suggested that the correlation of torus mandibularis with other clinical variables might make an interesting subject for further investigation.
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