Third Molar Polymorphism and Its Significance to Dental Genetics.

In the now broad field of human dental genetics, variations in the number of permanent teeth have attracted repeated attention. Indeed, of articles dealing with population frequencies, pedigree analysis, or estimates of hereditability, the greatest number have been concerned with missing or supernumerary teeth.'-4 It is not difficult to understand why such tooth number polymorphisms have preempted genetic attention. Missing teeth or supernumerary teeth command notice, for they are evident even to the unaided eye of the morphologically unsophisticated observer. Furthermore, if the propositus is a child or young adult, other siblings are generally available for study, and so (often) are the parents. Missing teeth, moreover, can be surveyed in archeological populations and in museum collections quite as readily as in living individuals. Thus the dental literature truly overflows with two-generation pedigrees of missing incisors, with estimates of the incidence of agenesis in various demographic and Mendelian populations and with reports of missing or supernumerary teeth in clinic patients, dental students, skeletal collections, and isolated remains. By and large, variations in the number of teeth have been treated as purely unrelated anomalies, with no theoretical justification given for any particular study. Except for highly inbred, hybrid, or trihybrid populations,'-7 where an increased proportion of agenesis may be expected,4' 8t I adventitious or practical, rather than theoretical reasons have dictated the study of missing teeth. They were studied simply because they were missing, or they were reported simply because they were observed. Little attention, in fact, has been given to the real possibility that tooth number polymorphisms in man are not isolated phenomena but bear a fundamental relationship to the size, development, and calcification timing of the dentition as a whole. Scant attention has been given to variations in tooth number as continuing evolutionary directions already perceived in the fossil record. And virtually no attention has been given to the mechanisms that must maintain tooth number polymorphism in man. Now except for supernumerary teeth like the fourth molars, variations in tooth number are not particularly rare; nor, for that matter, are they exceptionally common. The most variable of the teeth (the third molars, the lateral incisors, and the second premolars) range between 1 and 30 per cent in the prevalence of agenesis. Such frequencies hovering close to 5 per cent in general-are ideal for genetic investigations, being sufficiently high to minimize the size of the sample required, yet not so high …