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There is considerable variation between and within populations with regard to tooth size, age of eruption, congenitally missing teeth, and crown morphology. These differences are a reflection of the ongoing process of evolution. The genetic basis for this variation is best explained by a polygenic model of inheritance. Even though tooth morpholoyg and agenesis have discontinuous distributions - usually evidence of single gene transmission - they are also polygenic traits and are examples of "quasi-continuous" characteristics. These are traits which exhibit phenotypic discontinuity at the end of a continuous distribution. Postnatal conditions such as nutrition, disease, or climate seem to have little influence on normal dental variation. Most of the environmental factors which affect the dentition occur during the prenatal period. In particular the quality of the intrauterine environment appears to be most important. This suggests that good prenatal nutrition and medical care are needed for a normal and healthy dentition.