Genetic covariance structure of incisor crown size in twins

Geometric morphometric analysis of mandibular symphysis in Class III skeletal base

This study aimed to investigate the general mandibular symphysis (MS) shape variation among Class III skeletal base, using geometric morphometric analysis. Pre-treatment lateral cephalometric radiographs of 254 patients aged 11-40 years old, with Class III skeletal base (ANB <1°) and lower incisor angle (<99°) were included. Nine-landmarks with x and y coordinates were identified on MS using TPSDig2 software, then exported into Morpho J for shape and statistical analysis. Principal component analysis showed that three main shape dimensions with a total variance of 74.6% represented the majority variation of samples. Procrustes Anova showed the shape of MS in Class III skeletal base to be mainly influenced by gonial angle, incisor inclination and sex (P<0.0001). Canonical variate analysis showed that high gonial angle groups had significantly narrower and elongated MS whereas low gonial angle groups had wider, bulbous and rounded MS (P<0.0001). The ratio of alveolar part to basal part was 1:5 in low gonial angle and 2:3 in high gonial angle. Males had significantly taller MS with narrower B point area compared to females (P<0.0001). Retroclined incisors exhibited taller and retroclined alveolar parts (P<0.0001). The shape of MS in Class III skeletal base varied at the alveolar part, basal part or both and it is influenced by gonial angle, incisor inclination and sex. Hence, understanding the shape variation of MS is important to aid orthodontic treatment planning.

Bridging the gap: Using biological data from teeth to comment on social identity of archeological populations from early Anglo-Saxon, England

Human teeth are storytellers, in that, through analysis of their size and shape osteoarchaeologists are able to 'talk' to the dead and translate biological data into social meaning. This concept has been explored in parts of the world through investigations of biological similarity and kinship, but few have focused in depth on early medieval populations who emphasized the importance of family and kinship. This paper presents the results from four early Anglo-Saxon cemeteries which highlight the utility of dental metrics in identifying biological similarity within the skeletal assemblages. 5988 mesiodistal and buccolingual measurements were recorded from the identifiable permanent dentition of adult individuals from early Anglo-Saxon cemeteries in the UK counties of Cambridgeshire and Kent. Results from statistical hierarchical cluster analysis of dental metric data revealed that it was possible to identify individuals within the cemetery sites that were more similar to one another according to their dental metrics. This similarity was not attributed statistically to biological sex or shared familial environment, as similarity between individuals could be found between males and females and few significant differences were found across the sites sampled. It was found that tooth metrics provided a meaningful biological dataset from which current theories regarding the identity of Anglo-Saxon individuals and families could be refined and improved. These types of data are useful as building blocks which help to bridge the gap between social constructs and human skeletal remains in order to substantiate interpretations about past populations in more significant ways. This work supports the need for multidisciplinary approaches to bioarchaeological investigations of past people while highlighting the utility of human dentition to enhance such areas of study.

Relative tooth size, Bayesian inference, and Homo naledi

OBJECTIVES

Size-corrected tooth crown measurements were used to estimate phenetic affinities among Homo naledi (~335-236 ka) and 11 other Plio-Pleistocene and recent species. To assess further their efficacy, and identify dental evolutionary trends, the data were then quantitatively coded for phylogenetic analyses. Results from both methods contribute additional characterization of H. naledi relative to other hominins.

MATERIALS AND METHODS

After division by their geometric mean, scaled mesiodistal and buccolingual dimensions were used in tooth size apportionment analysis to compare H. naledi with Australopithecus africanus, A. afarensis, Paranthropus robustus, P. boisei, H. habilis, H. ergaster, H. erectus, H. heidelbergensis, H. neanderthalensis, H. sapiens, and Pan troglodytes. These data produce equivalently scaled samples unaffected by interspecific size differences. The data were then gap-weighted for Bayesian inference.

RESULTS

Congruence in interspecific relationships is evident between methods, and with many inferred from earlier systematic studies. However, the present results place H. naledi as a sister taxon to H. habilis, based on a symplesiomorphic pattern of relative tooth size. In the preferred Bayesian phylogram, H. naledi is nested within a clade comprising all Homo species, but it shares some characteristics with australopiths and, particularly, early Homo.

DISCUSSION

Phylogenetic analyses of relative tooth size yield information about evolutionary dental trends not previously reported in H. naledi and the other hominins. Moreover, with an appropriate model these data recovered plausible evolutionary relationships. Together, the findings support recent study suggesting H. naledi originated long before the geological date of the Dinaledi Chamber, from which the specimens under study were recovered.

Twins and the paradox of dental-age estimations: a caution for researchers and clinicians

The biological age difference among twins is frequently an issue in studies of genetic influence on various dental features, particularly dental development. The timing of dental development is a crucial issue also for many clinicians and researchers. The aim of this study was therefore to verify within groups of twins how dental development differs, by applying Demirjian's method, Mincer's charts of development of third molars and two of Cameriere's methods for dental age estimation, which are among the most popular methods both in the clinical and the forensic scenario. The sample consisted of 64 twin pairs: 21 monozygotic, 30 dizygotic same-sex and 13 dizygotic opposite-sex with an age range between 5.8 and 22.6 years. Dental age was determined from radiographs using the mentioned methods. Results showed that dental age of monozygotic twins is not identical even if they share all their genes. The mean intra-pair difference of monozygotic pairs was low and similar to the difference in dizygotic same-sex twins; the maximum difference between monozygotic twins, however, was surprisingly large (nearly two years). This should lead to some circumspection in the interpretation of systematic estimations of dental age both in the clinical and forensic scenario.

Mandibular anterior bony support and incisor crowding: Is there a relationship?

INTRODUCTION

The aim of this study was to test the null hypothesis that increased irregularity of the mandibular incisors is associated with a reduction in the alveolar support on cone-beam computed tomographic sections.

METHODS

From a sample of 1100 digital volumetric tomographs, 125 tomographs of subjects with Class I malocclusion (mean age, 21.6 ± 4.8 years) were selected for this study. An irregularity index was used to categorize these tomographs as having mild, moderate, or severe crowding. All tomographs were taken by using an iCAT (Imaging Sciences International, Hatfield, Pa) imaging device. The following parameters were measured on the sections corresponding to the 4 mandibular incisors with the iCAT software: height, thickness, and area of the entire symphysis; height, thickness, and area of the cancellous bone of the symphysis; and distance between the vestibular and lingual cortices. For the statistical evaluation, independent samples t test, analysis of variance, and the Tukey HSD test were used at an alpha level 0.05. The Pearson correlation coefficient and a simple linear regression were calculated to determine the relationship between mandibular anterior bony support and incisor crowding.

RESULTS

Almost all mandibular anterior bone measurements were greater in the male subjects than in the female subjects (height of the mandibular symphysis, P <0.001; cancellous bone height, P <0.001). Female subjects with mild crowding had higher values for cancellous bone height (P = 0.025) and vestibular cancellous bone thickness (P = 0.004) than did those with severe crowding. However, no differences were detected in the male subjects. Additionally, significant correlations were determined between incisor crowding and thickness of the mandibular symphysis, cancellous bone thickness, and the vestibular part of cancellous bone thickness in female subjects.

CONCLUSIONS

Significant relationships were found between the measures of mandibular incisor crowding and basal bone dimensions in female subjects. Except for the vestibular part of cancellous bone thickness, all mandibular incisor bone measurements were greater in the male subjects than in the female subjects.

Genetic, Epigenetic, and Environmental Influences on Dentofacial Structures and Oral Health: Ongoing Studies of Australian Twins and Their Families

The Craniofacial Biology Research Group in the School of Dentistry at The University of Adelaide is entering an exciting new phase of its studies of dental development and oral health in twins and their families. Studies of the teeth and faces of Australian twins have been continuing for nearly 30 years, with three major cohorts of twins recruited over that time, and currently we are working with twins aged 2 years old to adults. Cross-sectional data and records relating to teeth and faces of twins are available for around 300 pairs of teenage twins, as well as longitudinal data for 300 pairs of twins examined at three different stages of development, once with primary teeth, once at the mixed dentition stage, and then again when the permanent teeth had emerged. The third cohort of twins comprises over 600 pairs of twins recruited at around birth, together with other family members. The emphasis in this third group of twins has been to record the timing of emergence of the primary teeth and also to sample saliva and dental plaque to establish the timing of colonization of decay-forming bacteria in the mouth. Analyses have confirmed that genetic factors strongly influence variation in timing of primary tooth emergence. The research team is now beginning to carry out clinical examinations of the twins to see whether those who become colonized earlier with decay-forming bacteria develop dental decay at an earlier age. By making comparisons within and between monozygotic (MZ) and dizygotic (DZ) twin pairs and applying modern molecular approaches, we are now teasing out how genetic, epigenetic, and environmental factors interact to influence dental development and also oral health.

Genetic and Environmental Influences on Dentofacial Structures and Oral Health: Studies of Australian Twins and Their Families

Our studies of the teeth and faces of Australian twins commenced at the School of Dentistry, The University of Adelaide in the early 1980s. There are now over 900 pairs of twins enrolled in our continuing investigations, together with 1200 relatives. There are 3 main cohorts of participants. The first cohort comprises around 300 pairs of teenage twins for whom various records have been collected, including dental casts, facial photographs, finger and palm prints and information on laterality, including handedness. The second cohort comprises around 300 pairs of twins who have been examined at 3 stages of dental development from approximately 4 years of age to about 14 years: at primary, mixed, and permanent dentition (excluding 3rd molars) stages. The most recent study of tooth emergence and oral health, for which we are currently recruiting twins, will provide a third cohort of around 500 twin pairs aged from around birth to 3 to 4 years of age. Our broad aim in these studies has been to improve our understanding of how genetic and environmental factors contribute to variation in dental and facial features, and to oral health. We have also used our data to investigate aspects of the determination of laterality, particularly the fascinating phenomenon of mirror imaging. We plan to maximize the use of the longitudinal data and DNA we have collected, and continue to collect, by performing genome-wide scans for putative genetic linkage peaks for a range of dental features, and then to test for association between a series of likely candidate genes and our phenotypes.

Ethnic differences in dentofacial relationships of Turkish and Saudi young adults with normal occlusions and well-balanced faces

OBJECTIVES

The aims of the present study were (1) to determine ethnic differences in craniofacial dimensions between Turkish and Saudi populations and (2) to identify possible gender differences between males and females, based on a sample of untreated young adult subjects with normal occlusions and well-balanced faces.

METHODS

In total, 163 cephalometric radiographs were traced and evaluated to compare untreated adults of Turkish and Saudi ethnicity. The Turkish group comprised 86 subjects; 45 females and 41 males. The Saudi group comprised 77 subjects; 39 females and 38 males. For statistical evaluation, an independent-samples t-test was performed.

RESULTS

The Turkish sample had a more retrognathic maxilla and mandible (p < 0.001 for SNA and SNB) and a more vertical direction of facial development (p < 0.001), with Turkish males having more retrusive lips (p < 0.001). Distinctive ethnic differences were found in craniofacial structures between Turkish and Saudi young adults.

CONCLUSIONS

It is appropriate to consider these aesthetic differences when a Turkish or a Saudi patient is being evaluated during routine diagnosis and treatment planning.

Cone-beam computed tomography evaluation of relationship between tongue volume and lower incisor irregularity

The aim of this study was to evaluate the relationship between the tongue volume and lower incisor irregularity, using cone-beam computed tomography (CBCT), and to identify the possible gender differences. CBCT images of 60 patients between 16 and 36 years of age were selected from 1400 sets of images in the database. Tomography was carried out using iCAT® (Imaging Sciences International, Hatfield, Pennsylvania, USA) and segmentation was carried out by using Mimics 10.1 software (Materialise NV, Leuven, Belgium). The tongue volume was calculated by using the volume of the voxels from the scan and the number of voxels selected for a given mask. Lower incisor crowding was measured with the Little's irregularity index and divided into three groups: mild, moderate, and severe. Independent samples t-test, analysis of variance (ANOVA), and Tukey test were used at P < 0.05 level. Pearson correlation coefficients and linear regression model were calculated to determine the correlation between tongue volume and incisor irregularity. No significant gender dimorphism was found for the tongue volume (females: 28.13 ± 8.54 cm(3) and males: 31.02 ± 9.75 cm(3)). According to ANOVA, there was statistically significant difference in the tongue volume measurements among subjects with different levels of irregularity. Tukey analysis indicated that mild irregularity group (33.97 cm(3)) showed higher values for tongue volume than severe irregularity group (26.60 cm(3); P = 0.025). The relationship between incisor irregularity and tongue volume was evaluated for both genders and significant inverse correlation (r = -0.429; P = 0.029) was determined between lower incisor irregularity and tongue volume in males. In female group, no significant correlation was determined between tongue volume and incisor irregularity.

Genetic modeling of primary tooth emergence: a study of Australian twins

The aim of this study was to quantify contributions of genetic and environmental factors to variation in timing of emergence of the primary teeth in a sample of monozygotic and dizygotic twins, using univariate model-fitting approaches. The sample comprised 94 pairs of monozygotic twins and 125 pairs of dizygous twins, all of European ancestry, aged from 2-6 years. Tooth emergence timing was based on parental report, with a subset of data validated by clinical assessment. Heritability estimates for tooth emergence timing were generally high, around 90%, however estimates for the lower right lateral incisor and the lower canines were around 50%. These findings confirm a strong genetic influence on observed variation in the timing of emergence of the human primary teeth.

Genetic basis of tooth agenesis

Tooth agenesis or hypodontia, failure to develop all normally developing teeth, is one of the most common developmental anomalies in man. Common forms, including third molar agenesis and hypodontia of one or more of the incisors and premolars, constitute the great majority of cases. They typically affect those teeth that develop latest in each tooth class and these teeth are also most commonly affected in more severe and rare types of tooth agenesis. Specific vulnerability of the last developing teeth suggests that agenesis reflects quantitative defects during dental development. So far molecular genetics has revealed the genetic background of only rare forms of tooth agenesis. Mutations in MSX1, PAX9, AXIN2 and EDA have been identified in familial severe agenesis (oligodontia) and mutations in many other genes have been identified in syndromes in which tooth agenesis is a regular feature. Heterozygous loss of function mutations in many genes reduce the gene dose, whereas e.g. in hypohidrotic ectodermal dysplasia (EDA) the complete inactivation of the partially redundant signaling pathway reduces the signaling centers. Although these mechanisms involve quantitative disturbances, the phenotypes associated with mutations in different genes indicate that in addition to an overall reduction of odontogenic potential, tooth class-specific and more complex mechanisms are also involved. Although several of the genes so far identified in rare forms of tooth agenesis are being studied as candidate genes of common third molar agenesis and incisor and premolar hypodontia, it is plausible that novel genes that contribute to these phenotypes will also become identified.

Latent volumetric structure of the human brain: Exploratory factor analysis and structural equation modeling of gray matter volumes in healthy children and adults

Previous studies have investigated patterns of volumetric covariance (i.e. intercorrelation) among brain regions. Methodological issues, however, have limited the validity and generalizability of findings from these prior studies. Additionally, patterns of volumetric covariance have often been assumed to reflect the presence of structural networks, but this assumption has never been tested formally. We identified patterns of volumetric covariance, correlated these patterns with behavioral measures, and tested the hypothesis that the observed patterns of covariance reflect the presence of underlying networks. Specifically, we performed factor analysis on regional brain volumes of 99 healthy children and adults, and we correlated factor scores with scores on the Stroop Word-Color Interference Test. We identified four latent volumetric systems in each hemisphere: dorsal cortical, limbic, posterior, and basal ganglia. The positive correlation of the right posterior system with Stroop scores suggested that larger latent volumes are detrimental to inhibitory control. We also applied Structural Equation Modeling (SEM) to our dataset (n = 107) to test whether a model based on the anatomical pathways within cortico-striatal-thalamic-cortical (CSTC) circuits accounts for the covariances observed in our sample. The degree to which SEM predicted volumetric covariance in the CSTC circuit depended on whether we controlled for age and whole brain volume in the analyses. Removing the effects of age worsened the fit of the model, pointing to a possible developmental component in establishing connections within CSTC circuits. These modeling techniques may prove useful in the future for the study of structural networks in disease populations.

Size and shape analysis of human molars: comparing traditional and geometric morphometric techniques

Dental size and shape have been widely used to study biological relationships among human populations. Although several techniques have been proposed to quantify dental form, few attempts have been made to compare results obtained by application of different techniques. This work aims at comparing the information about size and shape of molar contours obtained from linear measurements, landmarks and semi-landmarks as well as evaluating the variation patterns among populations obtained by each method. The crowns of 35 permanent upper second molars belonging to archaeological samples from three regions of Argentina were analyzed. Buccolingual and mesiodistal crown diameters were measured and centroid size and crown index were used as size and shape descriptors, respectively. Likewise, four landmarks and 79 semi-landmarks were collected from the molar outline and relative warps (RW) analysis was performed on partial warps and uniform vectors; centroid size was estimated to summarize tooth size. The results indicate that size was consistently estimated by the three variable types. On the contrary, noticeable differences were found when the results of molar shape described by linear measurements were compared with those obtained using either landmarks or semi-landmarks. This study shows that considerable information about molar contour is added by using landmarks instead of crown diameters and that some morphological features, such as degree of cusp development, can only be captured by means of semi-landmark analysis. Finally, the interpretation of similarities among samples differs according to the selected description system.

The Uniqueness of the Human Anterior Dentition: A Geometric Morphometric Analysis

The analysis of bitemarks has a significant bearing on forensic odontology and has attracted an increasingly sophisticated array of techniques in its evaluation. Two postulates underlie all bitemark analyses: firstly, that the characteristics of the anterior teeth involved in the bite are unique, and secondly, that this uniqueness is accurately recorded in the material bitten. Here, we investigate the question of the uniqueness of the anterior dentition. To do this, we use geometric morphometric techniques based on landmark and semilandmark data. The incisor and canine occlusal surfaces of 50 randomly selected orthodontic casts of young individuals (17-20 years) of both sexes form the material for this study. We analyzed the sizes of these teeth by means of landmark and semilandmark analysis to calculate Procrustes distances between tooth outlines. In order to analyze shape variation among individuals, we carried out principal components analyses on the partial warp scores. These are derived from Partial Procrustes coordinates aligned by means of thin-plate spline decomposition based on the bending energy matrix. The results indicate that there is no sexual dimorphism in the shape of the upper or lower arches. Plots of centroid size and first relative warps show less superposition among individuals than in shape analysis. This means that, when the size and shape are considered together, the difference between arches increases. Procrustes superimposition between the two individuals located most closely (0.0444) and the two most separated (0.1567) along the first axis of relative warp analyses show that individuals are not only differentiated by the relative position of their teeth but also by their arch shape. In conclusion, it appears that the incisal surfaces of the anterior dentition are in fact unique.

Biological structure and health implications from tooth size at Mission San Luis de Apalachee

This study analyzes dental metric variation to examine the biological structure of the native population at Mission San Luis de Apalachee, a late 17th century mission located in the Apalachee Province of Spanish colonial Florida. Three topics are addressed: (1) comparison of tooth sizes among adult and subadults, (2) analysis of the bio-spatial structure of skeletons within the church area, and (3) comparison of phenotypic profiles of individuals interred within coffins in the ritual nucleus of the church: the altar region. Analyses indicate that subadults had smaller average tooth sizes than adults for the posterior dentition that was particularly evident in mandibular nonpolar molars and premolars. This disparity, also documented in two other mission populations, likely represents ontogenetic stress and resulting increased mortality among those most at risk for early death. Analysis of the spatial structure of graves failed to document biological structuring by side of the aisle or by burial row, although some gross differences were evident when front, middle, and rear church burials were compared. Individuals buried in coffins within the same row were phenotypically similar to one another. However, inter-row comparisons indicated lack of phenotypic similarity among all coffin interments. These analyses suggest maintenance of kin-structured burial for elites alone within the San Luis community.

Spanish colonial effects on Native American mating structure and genetic variability in northern and central Florida: Evidence from Apalachee and western Timucua

Standard population genetic analyses are implemented for a series of precontact and contact period samples from central and northern Florida to investigate changes in genetic variability and population affinity coincident with the establishment of Spanish missions during the 17th century. Estimates of F(ST) based on odontometric data indicate limited heterogeneity for the Apalachee samples, suggestive of some degree of within-group endogamy for this ethnic group prior to contact. This corresponds well with ethnohistoric reconstructions indicating that Apalachee were populous, partially linguistically isolated from its neighbors, and involved in persistent cycles of warfare with neighboring groups. Estimates of extralocal gene flow for the Apalachee samples indicate limited initial changes in the mating structure of these populations. After 1650, however, extralocal gene flow increases, consistent with evidence for dramatic population movements throughout northern Florida and increased Spanish presence in the province, particularly at the mission of San Luis. Inclusion of non-Apalachee outgroups does not increase estimates of genetic heterogeneity, as was expected based on ethnohistoric data. The pattern of genetic distances suggests a biological division between north and south Florida population groups, consistent with archaeological and ethnohistoric data, and similarly indicates some distinction between precontact and postcontact local groups. Differential extralocal gene flow experienced by pre-1650 Apalachee and Timucua populations suggests localized mission experience. The Apalachee, with large, dense populations, experienced limited initial changes in genetic diversity or mating structure. However, after 1650 they were apparently involved in a much more expansive mating network that may have included Spaniards and immigrant Native American groups to the region. These results are in contrast to the mission experience of the Guale Indians of the Georgia coast.

Correspondence between enamel hypoplasia and odontometric bilateral asymmetry in Australian twins

Four aspects of enamel hypoplasia of the maxillary central incisor and mandibular canine (hypoplasia presence, width, cumulative width, and crown position) were correlated with directional and fluctuating measures of bilateral odontometric asymmetry in a large panel (n = 950) of South Australian twins. Hypoplasia and asymmetry are thought to reflect general developmental disruption, but they show few correlations beyond the expected statistical type I error. This may relate to differences in their specific etiology, the composite nature of overall crown dimensions, a general lack of stress, and the extended period of formation of dental crowns. In contrast, asymmetry is marginally more detectable in a subsample separated according to hypoplastic teeth, suggesting that correspondence may be clearer in comparisons at the population rather than individual level. The most notable difference is the greater variability of asymmetry measures in hypoplastic individuals.

Population history of native groups in pre- and postcontact Spanish Florida: Aggregation, gene flow, and genetic drift on the Southeastern U.S. Atlantic coast

Evolutionary trends and population history and structure are discussed for a series of late prehistoric and historic-period skeletal samples from the Georgia coast and interior (the Guale). Phenotypic dental measurement data were collected for nine samples from the late prehistoric (AD 1200-1400) and historic (AD 1608-1702) periods and subjected to population genetic and statistical analyses. The primary trends were for an increase in tooth size through time, and for an initial increase in dental variability in the early historic period, followed by a subsequent decline in dental variability in the late historic period. Given the increasing stress levels, evidenced by previous bioarchaeological analyses (Larsen [2001] Bioarchaeology of Spanish Florida, Gainesville: University Press of Florida), an environmental explanation for the increase in tooth size is unlikely. It is proposed that the early historic period witnessed aggregation and gene flow with extraregional populations, possibly African slaves or nonlocal Native American population groups. The late historic period may have experienced significant loss of phenotypic variability due to genetic drift. In both time periods, the evolutionary mechanism increased average tooth size, with independent variance effects.Because microevolutionary trends obscure patterns of gene flow and population ancestry, the data were detrended following Konigsberg ([1990a] Hum. Biol. 62:49-70), and submitted to standard population genetic analyses (Relethford et al. [1997] Hum. Biol. 69:443-465). Analysis of the precontact samples in isolation (Irene Mound, Irene Mortuary, and an aggregate coastal sample) indicated little genetic microdifferentiation (F(ST) = 0.008), limited extralocal gene flow, and a small distinction between interior and coastal samples. The inclusion of the historic data dramatically increased variability levels (F(ST) = 0.019). The analysis of extralocal gene flow indicates that the late mission period experienced significantly less external gene flow, which is consistent with historic models that suggest the social organization of the Guale during this time period may have been significantly altered. Genetic distances also indicate a primary division between inland and coastal precontact samples and a maintenance of biological populations along the coast. In other words, the coastal, early historic, and late historic period samples are phenotypically homogeneous, supporting the notion that the mission populations were drawn from the local population base. The late mission period sample was also, however, more closely related to the interior samples. This may suggest that the late mission period population was an aggregate sample composed of both remnant interior and coastal population groups.

Genetic analysis of crown size in the first molars using SMXA recombinant inbred mouse strains

Tooth crown size may be determined by both genetic and environmental factors. The aim of this study was to identify quantitative trait loci (QTLs) affecting dental crown size and determine whether there is genetic independence between upper and lower teeth, using SMXA recombinant inbred strains of mice. Mesiodistal and buccolingual crown diameters (MD and BL, respectively) of the upper and lower first molars (M(1) and M(1), respectively) were measured. For each trait, mean values of substrains showed a continuous spectrum of distribution. Genome-wide scan detected QTLs exceeding suggestive threshold levels for MD of M(1) (chromosomes 7, 13, and 17), BL of M(1) (chromosomes 8 and 13), MD of M(1) (chromosomes 7 and 13), and BL of M(1) (chromosomes 3 and 15). These findings suggest that tooth crown size is controlled by multiple genes, and that there is some independence of genetic control between M(1) and M(1).

Matrix decomposition model for investigating prehistoric intracemetery biological variation

A matrix decomposition model for analyzing prehistoric intracemetery biological variability is presented. The model, based on the correlation between interindividual biological and burial distance matrices, provides an estimate of the number of distinct burial populations interred within a single cemetery, which effectively ameliorates identifiability problems typical of general variability analyses (Raemsch [1995] Ph.D. dissertation, SUNY at Albany). Unlike other methods for analyzing intrasite variability analysis (e.g., kinship analysis and microchronology), this method is not reliant on a priori subgroup definition. Assuming a kin-structured burial pattern, a residual matrix is defined based on the raw data matrices; the proportion of negative or zero residuals to the total number of residuals provides an estimate of the number of hidden subgroupings (lineages) within the cemetery. The comparative utility of the model is demonstrated on a series of protohistoric and historic period skeletal samples from the Georgia coast, where it demonstrated that the increase in phenotypic variability during the early mission period is the result of population aggregation at the missions and not necessarily genetic admixture between local and nonlocal groups.

Differential phenotypic variability among the Apalachee mission populations of La Florida: a diachronic perspective

Phenotypic variability is evaluated in a series of skeletal samples from the Apalachee region of Florida. Based on ethnohistoric evidence, several predictive models for changes in variability are generated. If variability decreases through time, this likely represents the effect of genetic drift in populations experiencing epidemic disease and population loss. If variability increases through time, this suggests that population aggregation or genetic admixture were primary factors shaping the Apalachee population during the mission period. Dental dimensions were collected from a series of precontact (pre-1500), early mission (AD 1633-1650) (San Pedro y San Pablo de Patale), and late mission (post-1657) (San Luis) samples from the Apalachee region and were subjected to univariate and multivariate variability analyses. The results indicate that the late mission San Luis sample was significantly more variable than the Patale or precontact samples; however, the Patale sample exhibited no significant variability change in comparison to the precontact population. This suggests that the missions initially effected limited change in genetic variability in the mission populations. However, San Luis was affected by either admixture or population aggregation to such a degree that the observed variation had increased beyond earlier levels. Given the limited historic evidence for population aggregation at this mission, and the comparatively large resident Spanish population, the increased variability may be indicative of admixture at this mission, and potentially at this mission only. Based on a limited data set, however, it appears that the mission period cannot be typified by a single evolutionary or historic process.

A study of occlusal variation in the primary dentition of Australian twins and singletons

In this study, we quantified the extent of variation in different occlusal features of Australian children of European descent with complete primary dentitions, but no permanent teeth present in the mouth. The study group consisted of 412 individuals, aged between 3 and 7 years, including 70 monozygous (MZ) twin pairs, 68 same-sexed dizygous (DZ) twin pairs, 11 opposite-sexed DZ twin pairs, and 114 singletons. Occlusal traits, including inter-dental spacing, incisal overbite and overjet, arch breadth and arch depth, were obtained directly from dental casts or indirectly from photocopies or impressions of the casts. Descriptive statistics summarised the data and indicated that distributions were similar to those published for other populations. Univariate genetic analysis, using the structural equation modelling package Mx, was carried out on the quantitative data using the normal assumptions of the twin model. Genetic modelling indicated that a model incorporating additive genetic (A) and unique environmental variation (E) was the most parsimonious for interdental spacing, overbite, overjet and arch dimensions. Estimates of heritability for interdental spacing ranged from 0.62 to 0.81. Estimates for overbite and overjet were 0.53 and 0.28, respectively, and estimates for arch dimensions ranged from 0.69 to 0.89. These results indicated a moderate to relatively high genetic contribution to observed variation.

Genetic analysis of deciduous tooth size in Australian twins

Investigations of permanent dental crown size in twins and family groups indicate a high degree of transmissible control, but little is known about the relative contributions of genetic and environmental factors to variation in size of the deciduous (primary) teeth. Here, maximum mesiodistal and buccolingual crown dimensions of maxillary and mandibular primary teeth were measured from dental models of 602 individuals, including 99 monozygous (MZ) twin pairs, 81 dizygous (DZ) same-sex pairs, 41 DZ opposite-sex pairs, and 160 singletons. Data were subjected to univariate genetic analysis with the structural-equation-modelling package, Mx using the normal assumptions of the twin model. A model incorporating additive genetic (A) and unique environmental (E) variation was found to be the most parsimonious for all tooth-size variables. Estimates of heritability for deciduous crown size ranged from 0.62 to 0.91. This study shows that variation in deciduous crown size has a strong genetic component, similar to that observed in the permanent dentition. Further studies are required to determine whether the underlying genetic mechanisms are the same for both deciduous and permanent teeth.

Increased tooth crown size in females with twin brothers: Evidence for hormonal diffusion between human twins in utero

In rodents, the position of a fetus in utero is associated with the expression of sexually dimorphic traits. This phenomenon has been explained by prenatal diffusion of sex hormones among litter mates. To test for such effects in humans, female-male twin pairs provide a natural experiment. The size of dental crowns is a sexually dimorphic trait which can be measured with a high degree of reliability. Thus, two crown diameters of 28 permanent teeth were recorded for 56 opposite-sexed (OS) and 242 same-sexed (SS) twin pairs, and 150 singletons. Comparisons of OS twins with SS twins and singletons within each sex reveal that OS females have consistently larger teeth (on average) than other females, while there is no consistent difference between OS and SS twin males. It is proposed that diffusion of sex hormones from male to female co-twins in utero may account for the increased tooth size in OS females. This study is one of the first to report such an effect on a morphological variable in humans. The finding that the maxillary canine, one of the most sexually dimorphic teeth, exhibits the least effect in OS female twins, suggests that prenatal sex hormone levels may have less impact on sexual dimorphism in the maxillary canines than in other permanent teeth. Am. J. Hum. Biol. 11:577-586, 1999. Copyright 1999 Wiley-Liss, Inc.