Asymmetry and discordance in Carabelli’s trait and the hypocone: do MZ and DZ twins point to common underlying epigenetic factors?
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Family and twin studies show there is a strong genetic component underlying the development of nonmetric tooth crown traits. If nonmetric dental traits are controlled by genes alone, then it is expected that (1) left and right antimeres should exhibit identical trait expressions, and (2) MZ twins, with identical genotypes, should display identical phenotypes. However, antimeres exhibit some asymmetry in trait expression and MZ twins likewise exhibit some level of discordant trait expression. Antimere asymmetry and MZ twin discordance suggest there are factors at work beyond genes that moderate crown trait expression, suggesting the possible action of common, underlying epigenetic mechanisms.This study looks beyond the classic “genetically controlled” paradigm of nonmetric crown tooth traits by examining the level of asymmetry and discordance in Carabelli’s cusp and the hypocone of 288 Australian monozygotic (MZ) and dizygotic (DZ) twin pairs. The level of asymmetry between antimeres is almost identical for MZ and DZ twins. Scores for discordance were similar for MZ twins but were consistently lower for all DZ twins. This study shows that measures of asymmetry in both twin types and discordance in MZ twins are almost identical, suggesting both are likely caused by epigenetic factors operating at the same magnitude. However, these factors may work independently on asymmetry and discordance as they do not covary in twins. As an ancillary test, birth weight data were compared to asymmetry and discordance scores in MZ and DZ twins. Neither absolute birth weights nor birth weight differences between twins were associated with levels of asymmetry or discordance. Birth weight, as a reflection of developmental perturbations and prenatal stress during pregnancy, was not reflected in the development of asymmetry or MZ twin discordance in Carabelli’s trait and the hypocone. Furthermore, sex of the twin pair had no correlation with the development of asymmetry of discordance.