Acknowledgements Prologue 1. Dental anthropology and morphology 2. Description and classification of permanent crown and root traits 3. Biological considerations: ontogeny, asymmetry, sex dimorphism and intertrait association 4. Genetics of morphological trait expression 5. Geographic variation in toot crown and root morphology 6. Establishing method and theory for using tooth morphology in reconstructions of late Pleistocene and Holocene human population history 7. Tooth morphology and population history Epilogue Appendices References Index.

The Anthropology of Modern Human Teeth: Dental Morphology and its Variation in Recent Human Populations

A growing body of information indipleiotropy and linkage of genes underlying cates that many, perhaps most, of the variation for two phenotypic characters. characters that interest evolutionists, ecolPleiotropy is a more pervasive cause of ogists, physiologists and behaviorists are genetic covariance than is linkage, partiepolygenic (Clausen et al., 1940; Clausen ularly if selection is weak (Falconer, 1960; and Hiesey, 1958; Cavalli-Sforza and Nagylaki and Crow, 1974). The imporBodmer, 1971; Ehrman and Parsons, tance of genic covariance is that it permits 1976; Wright, 1978). These characters prediction of correlated responses to seleccommonly show continuous frequency tion; if selection acts directly only on chardistributions that reflect segregation of acter x, how much evolution will be ingenes at many loci. In a landmark paper, duced in character y? (See DeFries and Fisher (1918) showed that an important Hegmann, 1970; Hegmann and DeFries, summary parameter could be established 1970a; Hegmann et al., 1973; Hegmann for such polygenic characters by measurand Boening, 1976; Pyle, 1978; Hegmann, ing and comparing the degree of pheno1979a, 1979b, for behavioral examples.) typic resemblance between various sets of Thus genic covariance can constrain the relatives. This parameter, the additive gedirection of evolution, just as heritability netic or genic variance, is the population can constrain its rate (Hazel, 1943; Crow property that is responsible for the pheand Nagylaki, 1976; Lande, 1979). Wright notypic resemblance between parents and (1968) has argued that pleiotropy may be offspring (it is totally responsible for this virtually universal: most genes probably resemblance in the absence of non-genetic affect more than one phenotypic characcauses of resemblance and if there is ranter. Thus many characters may be genetdom mating). Because it predicts the exically coupled and estimation of genic copected phenotypic mean of the next genvariance provides a means of determining eration from the phenotypes of parents, it the magnitude of genetic constraints imis the key genetic parameter that permits posed by pleiotropy. Like genic variance, prediction of response to selection in the genic covariance is sometimes expressed laboratory, barnyard, garden and in naas a ratio, the genetic correlation (the ratio ture (Fisher, 1930; Lush, 1945; Falconer, of genic covariance to the geometric mean 1960; Crow and Kimura, 1970; Mather of genic variances). The genetic correlaand Jinks, 1971; Lande, 19700). Additive tions describe correspondence or covarigenetic variance is sometimes expressed as ance in additive effects of genes on two a ratio to the phenotypic variance, that is different characters. as heritability, h 2 • This ratio represents Despite the pivotal importance of herthe proportion of phenotypic variance that itability and genetic correlation there are can be attributed to the additive effects of remarkably few estimates of these paramgenes. eters for natural populations. As Wright A second parameter, additive genetic or (1978, p. 323) put it, \"there have been relgenic covariance, is the bivariate analog atively few attempts at evaluation.\" In of genic variance. It reflects the degree of part this neglect is due to the difficulty of

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Behavioral variation in natural populations. i. phenotypic, genetic and environmental correlations between chemoreceptive responses to prey in the garter snake, thamnophis elegans.

Bilateral tooth measurements in twins were partitioned into three orthogonal contrasts, each associated with one degree of freedom, to estimate three parameters: discordance, asymmetry, and mirror imagery. The probability levels of the within-pair variance ratios were used to test for significance of these estimates. The results provided strong evidences for the existence of significant genetic determinants of almost all of the individual tooth dimensions, but little or no evidence for a genetic basis of asymmetry. The analysis gave no indication that monozygotic twinning was associated with an increased degree of either fluctuating asymmetry or mirror imagery, when compared to dizygotic twins. The data on monozygotic twins further suggested that for most variables examined, the increment of environmental discordance resulting from the twinning phenomena was greater than the developmental noise that caused asymmetry within individual cotwins.

A twin study of dental dimension. I. Discordance, asymmetry, and mirror imagery.

Recent investigations have shown that nongenetic, environmental factors can adversely affect dental growth and produce bilateral asymmetries in tooth size. When asymmetries do not favor either side, i.e., absence of directional asymmetry, the condition is termed fluctuating asymmetry. Fluctuating asymmetry of the mesiodistal and buccolingual dimensions of the total permanent dentition was compared among human skeletal populations which differ socio-economically and nutritionally. Odontometric data were collected from prehistoric hunters (Indian Knoll site), later aboriginal farming groups (Campbell and Larson sites), and a modern cadaver population (Hamann-Todd). The magnitude of asymmetry is expressed by the familiar correlation coefficient, r. The proportion, then, of intra-individual variation due to fluctuating asymmetry is equal to 1-r.

Fluctuating dental asymmetry: variation among skeletal populations.

A set of 20 morphological variants of the dental crowns and four characteristics of the jaws are tested for probable mode of inheritance using the complex segregation analysis method of Morton et al. (Am. J. Hum. Genet. 23:602–611, 1971). Models tested include three two-allele single-locus models (dominant, codominant, and recessive) and a model employing the polychotomized normal distribution of liability (an additive polygenic model), with transmissibility estimated via maximum likelihood. Most of the traits studied are observed using ordinal scales with several grades, and many are tested using more than one dichotomy of their scale. These multiple analyses allow for an examination of such factors as trait incidence on the results of the statistical analysis. The results of the analysis yield propositions of major genes for 13 of the 24 traits examined. Two traits give good evidence of being polygenic in origin. The remaining nine characters present methodological problems that do not allow for a definite conclusion on their mode of inheritance at this time. The ability to test varying levels of transmissibility in the polygenic model allows for an estimation of the percentage of trait variance determined by familial factors. Estimates of transmissibility for all characters examined range from 0 to 1, with a mean of 0.36. These findings may suggest a large environmental role in the development of dental crown morphology. However, the possibility exists that difficulties in the ability to classify the expression of certain traits consistently result in overestimates of the environmental influences on the development of those characters.

Complex segregation analysis of dental morphological variants

Genetic studies of tooth size factors in Pima Indian families

The mechanical tedium and errors resulting front using manual calipers for tooth measureruents has been largely overcoine through developmnent and twxo years' experience in the use of an inexpensive, semoiau tomatic, electronic system. The systerin (illustration) consists of a linear motion potentionituter* mounted on a specially constructed calipert and excited lby a constant voltage ( 18 volts, DC) power supply. Variable potentiometer voltage output, reflecting caliper mnoxvement. is connected to a digital voltimetert calilirated to display voltage in millimeters. By pressing a foot switch, the measurement is recorded on a digital printer.§ A button switch for printer spacing, needed for missing teeth, and a cylindrical standard used for instrumnent calibration are not illustrated. Mesiodistal and buccolingual dentitiost measuremcnts xxere performed on 28 permanent teeth in each of 635 casts. Approximately 250 of these casts wx ere measured xilth a standard vernier

R H Potter

Papers

Genetic studies of tooth size factors in Pima Indian families

Semiautomatic digital printing caliper for tooth measurements.