Third Molar Polymorphism and Its Significance to Dental Genetics

¹ Fels Research Institute, Yellow Springs, Ohio

Third molar agenesis is a relatively common polymorphism occurring in 16 per cent of the southwestern Ohio white population. Though previously treated as an independent anomaly, this number reduction is unquestionably related to agenesis of other teeth, to delayed formation timing of the remaining posterior teeth, to differences in tooth sequence polymorphisms, and to delayed timing and movement of the third molar tooth itself in the siblings of affected individuals. The association between third molar agenesis and reduction in the number of other teeth fits the hypothesis of a field of variable intensity, which, in its greatest degree of expression, eliminates all 4 third molar teeth and a maximum number of other teeth. The association between third molar reduction and developmental delay in the dentition is susceptible to at least 2 hypotheses, one involving pleiotropic manifestations of a single gene and the other involving two independent genes, the first favoring developmental suppression and the second affecting formation timing. While the degree of independence between these 2 phenomena may show which hypothesis is correct, the possibility of closely linked genes must also be considered. In this latter event, the monogenic and polygenic hypotheses would be operationally identical.

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