ABSTRACTThe semidominant gene ‘crooked’ (Cd) in the mouse produces anomalies of the axial skeleton (resulting in a crooked tail), microphthalmia and dental abnormalities, including small molars with simplified cusp patterns that are equivalent to patterns passed through during normal morphodifferentiation. A series of embryonic litters from Cd/+×Cd/+ matings was used to investigate the embryological basis for the dental abnormalities. Micro-phthalmic embryos were classed as Cd/Cd, and their most normal litter mates were selected as controls (+ /+ or Cd/+). An additional set of control embryos came from the inbred strain CBA/Cam (+/ + ). Serial sagittal sections of the heads of these embryos were examined microscopically, and the maximum anteroposterior diameters of the developing upper and lower first molars were measured. Reduction in the rates of growth and morphodifferentiation of Cd/Cd first molars, relative to those of litter mate controls, was associated with the appearance of an adjacent abnormal proliferation of the dental lamina. Some proliferations in older embryos showed signs of early tooth germ formation, but many were seen to have regressed and no examples of supernumerary teeth have been found in Cd/Cd adults. Small size of Cd/Cd molars may therefore result from competitive inhibition of molar growth by a transient abnormal laminal proliferation, and Cd/Cd cusp patterns from the relatively premature onset of hard tissue formation during normal but retarded sequences of morphodifferentiation.