Gaining New Insights into How Genetic Factors Influence Human Dental Development by Studying Twins

Many previous attempts to quantify the contribution of genetic factors to human dental variation using the classical twin design have been based on untested assumptions that lead to unreliable estimates of heritability. We have applied structural equation modelling to several different dental phenotypes in a sample of over 600 pairs of Australian twins, enabling the goodness-of-fit of the data to be tested against genetic models incorporating different components of genetic and environmental variance. Our results indicate that the contribution of additive genetic effects to phenotypic variation differs considerably between different dental traits. Heritability estimates for intercuspal distances of molar teeth and for incisal overbite and overjet are low to moderate in magnitude, whereas heritabilities for overall molar crown size and arch dimensions are moderate to high. We propose that after formation of the enamel knots during odontogenesis, the emerging pattern of molar cusps results from a cascade of local epigenetic events, rather than being under direct genetic control. Variation in molar crown size is explained best by a model incorporating additive genetic effects, as well as environmental influences that are both unique and common to co-twins. These environmental influences presumably operate in utero during the early stages of molar odontogenesis prior to crown calcification. The relatively low heritabilities noted for occlusal traits are consistent with the importance of masticatory activity and muscle function in determining the interrelationships between teeth in opposing dental arches. We believe that well-designed studies of twins, coupled with modern genome-scanning approaches, offer great potential to identify key “dental” genes and to clarify how these genes interact with the environment during development.