Teeth fall into a series of continuous morphogenetic fields (incisor, canine and molar) within which gradients of size and shape exist.Tooth agenesis may occur in regions where the usual capacity for tooth formation is reduced or lacking. In general, reduced tooth size, reduction in complexity and total absence are different degrees of the same thing. Both missing or reduced teeth often occur at the periphery of a morphogenetic series. Among modern humans, third molars are the most commonly missing or reduced tooth type, followed by upper lateral incisors, upper or lower second premolars and lower first incisors. The proportion of a population with one or more missing third molars ranges from almost none to 30% and while clearly genetically determined, this strongly suggests genetic drift has a large part to play. Changes in tooth number have been reported in 5% of Gorilla, 4.3% of Pan and 8,5% of Pongo museum specimens. Agenesis of one or more third molars is very rare in great apes but most rare in Gorilla. Agenesis or reduced peg-shaped upper lateral incisors and lower second premolars all occur in great apes.
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