The genetics and development of fused and supernumerary molars in the rice rat.

Analysis of breeding records suggests that the occurrence of fused and supernumerary molars in the rice rat, associated with lower body weight than normal and with reduced fertility, is dependent on a single autosomal recessive gene subject to background modification. Molar fusion, which is preceded by stripping of the external enamel epithelium from the interdental lamina, may involve the first two molars, or all three molars of the normal series in either jaw. The supernumerary is a posterior tooth, developing after the three molars of the normal series in either jaw. Supernumerary development usually occurs with fusion of the molars of the normal series, but occasionally a supernumerary may be present in the absence of fusion. Other examples of association between fusion, subsequent to separation of the external enamel epithelium from the interdental lamina, and supernumerary tooth development are cited. This association suggests that there may be a single common attribute of the dental lamina predisposing to epithelial stripping and to laminal hyperactivity.