Variance of occlusion traits in twins

To reexamine the prevailing concepts of the role of genetic and environmental factors on dental occlusion, individual occlusal traits were studied in 164 pairs of twins. The measured variables were overjet, overbite, crossbite type and extent, buccal segment relationship type and extent, anterior and posterior teeth malalignment, spacing, treatment priority index scores, and the principal components derived from these measures. No association of twin type with the mean of any of the traits could be demonstrated. Further analysis of variance revealed separate sources of twin variance for the set of occlusal variables studied. We were able to detect a significant genetic source of variation for overbite and spacing, and no variance inequality between monozygotic and dizygotic twins. For the majority of the trains (overjet, crossbite, buccal segment relationship, malalignment, and treatment priority index scores), tests of homogeneity of total variance showed inequality between zygosities. The total among-pair and within-pair mean squares were larger for dizygotic than for monozygotic twins. We interpreted the results as evidence for greater environmental influences on the two types of twins. In addition, environmental covariance appeared to be larger for monozygotic than for dizygotic twins with respect to crossbite. In cases where within-pair genetic variance estimates were highly significant, removing the bias due to unequal environmental effects on the two types of twins led to results of no significant genetic variance. These results indicate a further need to explore the sources of environmental variation contributing to the difference. Genetic models that may be used to gain insights into the sources are discussed.