AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 52315-321 (19801 Discrete Dental Trait Asymmetry in Mexican and Belizean Groups R.M. BAUME A N D M.H. CRAWFORD Laborntory of Biological Anthropology, L'niuersrt.v of Kansas, Lawrence, Kansas 66045 KEY WORDS Discrete dental traits, Mexico, Belize, Asymmetry ABSTRACT Discrete dental trait asymmetry was scored on four Mexican Indian and two Afro-Belizean groups. The Mexican populations show significantly higher mean asymmetry than the two populations from Belize even though the Belizean groups have larger teeth. Point biserial coefficients ofcorreiation between asymmetry of discrete traits and tooth size is low and in most cases not significant. This study indicates that populational and ethnic differences exist in the asymmetry ofdiscrete traits, suggesting that these traits may be used to investigate the etiology of dental asymmetry, and the relative roles of genetics and environment. Human populations have been shown to differ in the magnitude of metric fluctuating dental asymmetry (Bailit et al., '70). Although the determinants of this asymmetry remain unclear (DiBennardo and Bailit, '78; O'Rourke and Crawford, '791, environmental stress has been suggested as a contributing factor (DiBennardo and Bailit, '78). Varying degrees of discrete dental trait differences have been observed a m o n g populations ( B a u m e a n d Crawford, '78) however there has been little investigation of populational differences in regard to frequencies of discrete dental trait asymmetry. The purpose of this study is to test whether t h e asymmetry of discrete dental traits is observed differentially between populations and ethnic groups. This study is comprised of four related Mexican communities and two related ethnic groups from Belize. The four Mexican Indian groups a r e the historically and genetically related communities of C u a n a l a n , S a n Pablo del Monte, Tlaxcala City and Saltillo. Cuanalan and Saltillo were originally settled in the 17th century by transplanted Tlaxcaltecans. S a n Pablo is a n Indian population from the Valley of Tlaxcala, while the City of Tlaxcala is a mixed (Mestizo)community from the same valley. The trihybrid populations of Mexico a r e primarily Indian with some Spanish and little African ancestry (Crawford et al., '79). Caribs and Creoles, the two Belizean ethnic groups, are hybrid populations. Caribs stem 0002-9483/80/5203-0:~15$~1 70 (1 1980 ALAN K. LISS, INC from escaped West African slaves and Carib Indians. Phenotypically the Indian component is not prominent and admixture estimates suggest that about 80% of the Carib genome is West African in origin (Crawford '79). Creoles are for the most part hybrid African and European. During recent times, little intermarriage has been reported, despite the fact that t h e two groups are highly mobile (Gonzales '69). MATERIALS AND METHODS Approximately 700 dental casts were collected by anthropologic field teams in t h e four Mexican communities between 1969 and 1972. In Belize 282 dental casts were collected a t two locations, Dangriga and Punta Gorda, during the summer of 1976. For all samples, alginate impressions were made directly from dentition of individuals who were receiving medical care a t public health clinics. The casts were then poured in dental stone. Nine maxillary and seven mandibular discrete traits were scored on each cast. The discrete dental traits scored and the scoring techniques used in this study are summarized in Table 1. Only permanent teeth were utilized, and the age and sex of each subject were recorded. Not all traits could be scored on all casts, either because of agenesis or excessive wear. Inter-observer error is of no significance in this study since all casts were scored by one Ileceived Octnher :10. IYiK, accepted Mxy :11. 1979 315 R.M. BAUME AND M.H. CKAWFOKD 316 researcher. Asymmetry of discrete traits was scored as present when different degrees of expression were found on corresponding teeth on each side of the dental arch. There were few cases in which a trait was present on a tooth but absent on its antimere. More commonly asymmetry varied by only one or two degrees of expression. Therefore all occurrences of asymmetry were weighted equally. A discrete trait was scored as present on a tooth if it was expressed to any degree. For t h e maxillary cusp number, the affected teeth exhibited four cusps including the "3 +" classification. Affected mandibular molars showed the "Y" cusp pattern. I n the case of rotation of incisors, any rotated incisor whether winged or counter winged was scored as affected by asymmetry when its antimere did not match. When a random sample of casts was rescored several months later, a high concordance of scores for each trait was observed. A fuller discussion of concordance and repeatability of scoring techniques can be found in Baume and Crawford ('78). At the completion of the scoring process Kendall's coefficient of rank correlation analysis (Statistical Package for the Social Sciences, University of Kansas Computation Center, Version 6.02B01) was applied to the data in order to determine if there was a high degree of significance of association between the left and right quadrants of the dental arch. Percentage of asymmetry was calculated for each trait in each of the samples. It was computed by dividing the total number of casts scored for each trait by the number of cases of asymmetry present for each trait. A statistical test for the significance of differences between two percentages, based on arcsine transformations (Sokal and Rohlf, '69) was used to determine the independence of asymmetrical frequencies between the Mexican and Belizean groups. Lastly, point biserial coefficients of correlation (Walker and Lev, '53) were computed to test whether either of two classes (presence or absence of asymmetry) of the discrete dichotomous variable is correlated with tooth size. For this test the samples were divided by sex, since sexual dimorphism of tooth size is significant. RESUL'M Kendall's rank correlation coefficient for each trait in each sample along with sample size, is shown in Table 2. The percentage of asymmetry for each trait in each sample is shown in Table 3. These tables indicate that there is a low incidence of asymmetry and a high correlation between each side of the dental arch. For example, Table 2 shows that more t h a n half of t h e traits in each sample have correlations above 0.7. Moreover, all traits in all populations are significantly correlated between sides of the dental arch at the ,001 level. Table 3 indicates that asymmetry ranges from 0 to 31.6'%, with mean average asymmetry among the groups from Mexico ranging from 10.46% to 11.35% and among t h e Belizean groups asymmetry averages 8.33%. Moreover, when the mean percentage of asymmetry for the Belizean groups is tested against the mean for the Mexican groups, as indicated in Table 3 , a highly significant difference is observed. These differences are not a function of the small Creole sample size since the significant difference remains (with the exception of San Pablo which is close to significant) when the larger Belizean group, the Caribs, is tested against t h e Mexican groups independently of t h e Creoles. Because there were no significant differences in discrete traits between males and Tohle I . Methods /hr Scoring Discrete Dental Trczrts Trait Shovelling l ' , I' Protostylid M, and M, Carahelli's cusp M' and M' Maxillary cusp number MI and M Mandibular cusp pattern M' and M L Maxillary and mandibular canine ridges I ' , I' and I , , 1: incisor rotation Scoring Technique 5 degree3 of expression on central incisors. barrel, double, marked, moderate trace, absence 4 degrees of expression on first molar. pit, groove, non-free tip, free tip, ahsence 4 degrees of expression on first molar. absence, pit, groove, non-free tip, free tip 4 type classification system on first and second molars: 4 + , 4, 3 + , 3 4 type classification system on first and second molars. Y5, Y4, t 5, t 4 4 degrees of expression on upper and lower canines: absence of ridges, marginal ridges medial ridge, both marginal and medial ridges 3 degrees of expression on upper and lower incisors. winged, straight, counter-winged Reference tDahlberg, '56 with modifications) ~Uahlherg,'56 with modifications, ~Dahlberg,'56 with modifications1 (Uahlberg, '56) tDahlherg, '56) [Morris, '65 with modifications) I Dahlberg, '631 Sample sizes in parentheses. xCoefficient data cannot h e computed Maxillary teeth Shovelling I' Shovelling I' Rotation I1 Rotation I' Canine ridges Cusp number Mi Cusp number M2 Carahelli's cusp M1 Carabelli's cusp M' Mandibular teeth Rotation I , Rotation I, Canine ridges Cusp pattern M , Cusp pattern MZ Protostylid M I Protostylid M, _ _ x; 0.53 1238) 0.44 (238) 0.83 1225) 0.98 ! 1491 0.91 (531 0.85 ! 155) 0.99 (228) 0.98 (2281 0.91 (2311 0.40 ( 2 3 2 ) 0.86 (214) 0.67 (188) 0.88 (63) 0.79 (1861 1.00 (58) Caribs __ _ _ _ _ _ _* 0.43 (72) 0.47 (70) 0.75 150) 0.68 (47) 0.73 (9) 0.70 (57) 0.85 1311 0.60 (311 0.69 1251 0.97 ! 151 _ _ 0.93 (25) 0.52 (81 0.93 (72) 0.76 (72) 0.73 (72) 0.66 (721 0.78 (65) 0.70 (58) 0.80 (12) 0.81 158) __ Cuanalan 0.96 (29) 0.75 (301 1.00 (30) 0.71 (31) 1.00 (311 0.60 (251 Creoles Table 2. Kendall's rank correlation coefficient I T ) 0.35 (1071 0.39 (106) 0.70 (78) 0.79 I671 0.88 (43) 1.00 t75) 1.00 (451 0.97 ! 104) 0.90 (1031 0.96 (107) 0.62 (107) 0.87 (95) 0.65 (91) 0.85 (481 0.82 (91) 0.53 (44) Saltillo 0.69 (85) 0.45 (821 0.66 (74) 0.90 t70) 0.93 135) 1.00 173) 1.00 (41) _ _* 0.88 (86) 0.83 (84) 0.77 (95) 0.57 (84) 0.74 (77) 0.88 (71) 0.82 (36) 0.85 (73) San Pablo - -* 0.70 (741 0.50 (74) 0.70 (60) 0.86 (49) 0.88 (22) 0.87 (49) 0.99 (73) 0.89 (66) 0.79 (74) 0.69 (71) 0.81 (72) 0.93 t64J 0.83 (31) 0.87 (67) 0.76 (321 Tlaxcala 'P 0 05 - 0 4750 Maxillary teeth Shovelling I' Shovelling I' Rotation I' Rotation I' Canine ridges Cusp number M' Cusp number M' Carabelli's cusp M' Carabelli's cusp M' Mandibular teeth Rotation I , Rotation I2 Canine ridges Cusp pattern M, Cusp pattern M, Protostylid M, Protostylid M I Mean asymmetry Differences between percentages Caribs and other groups Total Belize and Mexico = 0.4998 11.06 0.4834* 7.93 0.1844 8.72 - - - 19.44 16.25 10.00 6.38 0 1.75 - - 6.45 9.68 12.00 6.67 4.16 20.83 11.11 8.33 15.38 7.00 17.00 17.24 Cuanalan 3.45 10.00 0 6.45 0 12.00 12.50 16.00 Creoles - 9.24 16.39 10.67 1.34 5.66 0.65 1.75 10.97 2.60 14.67 9.35 15.43 11.11 20.97 0 Caribs ~ 0.4972- 15.88 20.56 24.35 7.46 2.33 0 0 11.35 2.88 19.41 11.21 10.28 12.63 10.99 10.42 28.57 4 54 Saltillo ~ Table 3. Percentage asymmetry for each trait for euch group, average asymmetry, and differences when Belize is compared to each Mexican sample. ~~ 0.4545 10.46 0 0 9.41 19.51 22.97 11.43 5.71 - 1.16 13.09 10.53 14.28 14.29 5.63 11.11 17.81 San Pablo ~~ 0.4846* 11.16 - 9.45 18.33 31.66 2.04 4.55 2.04 9.58 10.60 8.10 8.45 19.44 6.25 9.67 20.89 6.25 Tlaxcala 5 e DENTAL. TRAIT ASYMMETRY females, both sexes were scored as a single group. Finally the relationship between tooth size and discrete trait asymmetry was tested utilizing the point biserial correlation coefficient method. The results are shown in Tables 4 and 5. These tables indicate that for molar pattern and number, and for shovelled incisors there is, for the most part, a negative correlation between tooth size and asymmetry. However in all but one case the correlation is not significant. For Carabelli's cusp and protostylid this negative correlation is viewed across the board. Although no pattern is indicated for canine ridges, rotation of incisors shows a clear positive correlation in each sample. Most of these are not significant, however, most values are close to being significant. It is interesting to note that for the largest sample, the Caribs, the asymmetry of most traits is significantly correlated with tooth size. Therefore, the lack of significance in the smaller groups may in fact be an artifact of small sample size. Positive correlations of tooth size and asymmetry may be explained by the nature of the trait itself. For example, tooth rotation may result from crowding of teeth in the dental arch; since larger teeth are more apt to be crowded and therefore mechanically rotated, it is expected that large teeth are positively correlated with rotation. DISCUSSION The high degree of significance of association between left and right quadrants in this study suggests that the genetic factors influencing discrete traits may be the same for both sides of the dental arch, as has been suggested by Green ('67).However, the expressivity of the trait may vary so that its ultimate expression is influenced either by local environmental conditions within the jaw andlor by intrauterine developmental factors. The significant populational differences between Belize, which has mostly African ancestry and the Mexican samples which are predominantly Indian and Mestizo, suggest that there are different genetic and environmental components contributing to the final phenotypic asymmetry for each of the groups. The problem in identifying the causes of dental trait asymmetry is that information concerning interactions of environmental and genetic mechanisms is lacking. Genetic factors cannot be completely ruled out in regard t o 319 dental trait asymmetry for two reasons. First, since there is an abundance of evidence that discrete dental traits are at least in part under genetic control (Goodman, '65; Alvesalo et al., '75) there is reason to suspect that discrete trait asymmetry may also be under genetic control. As Gruneberg ('65) has shown, nonmetric dental traits in mice are basically under genetic influence and can be expressed as continuous variables. The presence of a trait, then, may result when a genetically determined threshold point on the continuum is exceeded. If genetic influence is strong and the same genes control both sides of the dental arch, then there should be a high frequency of bilateral expression, while greater environmental influence should result in greater asymmetry (Bailit et al., '70). In this study, Table 3 shows that asymmetry in the Mexican and Belizean teeth is relatively low, indicating some genetic influences. Second, in a study by O'Rourke and Baume ('77)on the same dental casts, the Belizean teeth, which show less asymmetry in this study, were found to be significantly larger than the teeth of the Mexican groups, which show more asymmetry. Therefore, since the genetic role in determining tooth size is well documented (Goodman,'651, a negative correlation between tooth size and asymmetry should not be discounted. Moreover the point biserial coefficient ofcorrelation test indicated that for the discrete traits-shovelling of incisors, Carabelli's cusp, protostylid, and molar cusp number and pattern-such a negative correlation does in fact exist. Yet, environmental influences on the dentition cannot be ruled out either, since they can result in a variety of interactions with genotypes, culminating in a wide range of phenotypes. For example, the expression of Carabelli's cusp has been shown to increase when flouride is ingested during tooth formation (Cox et al., '61). Moreover, Alvesalo et al. ('75) found a low heritability for Carabelli's cusp. In addition, Bailit et al. ('70),have indicated that fluctuating dental asymmetry is greatest in populations experiencing the greatest environmental stress. CONCLUSION Studies of metric dental asymmetry have concentrated on environmental and developmental stress as a causal agent. The dentition begins to form and develop during the prenatal period. However, DiBennardo and Bailit ('78)found the evidence for a positive relation- = = insufficient data or no affected cases sipificant a t 0 05 level - - = = insufficient data or no affected case siplificant nt .05 level Shovelling I' Shovelling I' Rotation I' Rotation I' Rotation I, Rotation IL Canine ridges C' Canine ridges C , Cusp number M' Cusp number M' Cusp pattern M I Cusp pattern ML Carabelli's cusp M' Carabelli's cusp M' Protostylid M I Protostylid M, ' - Shovelling I' Shovelling I2 Rotation I' Rotation I' Rotation I , Rotation I, Canine ridges C ' Canine ridges C , Cusp number M' Cusp number M' Cusp pattern M , Cusp pattern M, Carahelli's cusp M' Carahelli's cusp M' Protostylid M , Protostylid M, - - -0.2327 - - -0.1004 -0.0950 0.2410 0.2700 0.2173 0.3125 0.1331 -0.1165 -0.1856 Tlaxcala -0.2126 -0.2866 - -0.0824 - - - - - - - -0.1062 -0.3100 - -0.0913 0.1028 - 0.1335 0.0731 -0.2299 - 0.1731 0.1187 0.2076 - -0.1940 -0,1602 0.2493 0.1474 0.2243 0.1142 0.0960 -0.1201 -0.1941 -0.1894 - Tlaxcala -0.1564* 0.2261* O.285EVF 0.2055" 0.1875" -0.1298 -0.0289 -0.1962 Creoles _ ~ - - -0.2993 - - -0.1178 - - -0.2340 - - -0.2139 -0.1302 0.0438 0.2667" 0.282P 0.3004* 0.1949 -0 1059 -0.0973 -0.1974 San Pahlo -0.1140 - -0.1703 - - - - -0.1826 - -0.1478 -0.3088 -0.3173 - -0.1820 -0.2268 0.3110 0.2688 0.3216 0.3047 -0.0972 0.1319 -0.1564 - Cuanalan - - -0.1055 0.0887 0.3145* 0.2921" 0.2659* 0.2295 0.1363 0.1020 -0.131 1 Saltillo - -0.2436 - -0.1979 - - -0.1429 -0.0969 - -0.0998 0.2551 0.2121 0.1008 0.1881 -0.1047 0.0748 -0.1242 - _ - _ -0.173 1 0.2930 0.2308 0.3078 0.1988 0.1113 0.0847 0.1048 _ - _ -0.1550 0.2873 0.2978 0.2456 0.2854 -0.1274 -0.0935 0.0913 ~ Cuanalan - Saltillo ~ San Pablo Table 5. Point hrserial correlation coefficient (females) - -0.2626 - 0.1080 -0.1716 - 0.2367'0.1973 0.0702 - -0.2136 -0.1362 Creoles Caribs - -0.356@* - -0.1690 -0.2237 -0.1009 0.2774" 0.3244' 0.2586 0.2622 -0.1486 -0.2376 -0.2189 Caribs Table 4 . Paint hrserial correlation coefficrent(males) 0 P U 5 0 ?J % z U > z E K ?J 0 t o w DENTAL TRAIT ASYMMETRY ship between individual asymmetry and prenatal stress in humans to be equivocal. In this study the gross physical and ecologic environments differ greatly between Belize and Mexico, as well as between each of the Mexican groups, yet mean dental asymmetry clusters together in Belize and Mexico. This suggests that asymmetry may in fact be influenced more by prenatal or intrauterine environmental stress, than by ecological considerations. In addition, this stress may be greater in Mexico than in Belize. ACKNOWLEDGMENTS Dennis O’Rourke and Seishi W. Oka collected some of the dental casts used in this study. This research was supported in part by National Institute of Dental Research grant DE04115-01 and Career Development Award KO4 DE028-0 1. LITERATURE CITED Alvesalo, L., M. Nutila, and P. Protin !1975) The cusp of Carabelli. Acta Odontol. Scand., 33:191-197. Bailit, H.L., P.L. Workman, J.F. Niswander, and C.J. MacClean (1970)Dental asymmetry as an indicator of genetic and environmental stress in human populations. Hum. Biol., 42:62&638. Baume, R.M., and M.H. Crawford (1978) Discrete Dental Traits in Four Tlaxcaltecan Mexican Populations Am. J. Phys. Anthropol., 49: 35 1-360. Cox, G.J., S.B. Finn, and D.B. Ast (1961) Effects of flouride ingestion on the size of the cusp of Carabelli during tooth formation J. Dent. Res., 40:39.%395. Crawford, M.H., R. Lisker, and B.P. Briceno (1976) Genetic 321 micrcdifferentiation of two transplanted Tlaxcaltecan populations. in: The Tlaxcaltecans, M.H. Crawford, ed. Publications in Anthropology, No. 7, University of Kansas, Lawrence. Crawford, M.H. (1979)Population structure of Black Caribs. In: Population Biology of Black Caribs, M.H. Crawford, ed. Publications in Anthropology University of Kansas, Lawrence (manuscript) Dahlberg, A.A. (1963) Analysis of American Indian Dentition: Dental Anthropology MacMillan, New York. Dahlherg, A.A. (1956) Materials for the establishment of standards for the classification of tooth characters, attributes and techniques in morphological studies of the dentition. Zoller Laboratory of Dental Anthropology. University of Chicago Press, Chicago. DiBennardo, R., and H.L. Bailit (1978) Stress and dental asymmetry in a population of Japanese children Am. J. Phys. Anthropol., 48:8%94. Gonzales, N.L. ( 1969) Black Carib Household Structure. University of Washington Press, Seattle. Goodman, H.O. (1965) Genetic parameters of dentofacial development J. Dent. Res. 44.176184. Green, D.L. < 1967) Genetics dentition taxonomy. University of Wyoming h b l . , 33:9%168. Gruneberg, H. (1965) Genes and genotypes affecting the teeth of the mouse J. Embryol. Exp. Morph., 14: 137-156. Morris, D.H. (1965) The Anthropological Utility of Dental Morphology. Unpublished Ph.D. dissertation, University of Arizona, Tuscon. ORourke, D.H., and R.M. Baume 11977) Dental variability among the Black Caribs of Belize. Paper presented a t the American Anthropological Association MeetingsHouston, Texas, December 1977. O’Rourke, D.H., and M.H. Crawford !1979)Fluctuating dental asymmetry in four populations of related genetic background Am. J. Phys. Anthropol. (In press). Sokal, R.L., and F.J. Rohlf (1969)Biometry. W.H. Freeman and Co., San Francisco. Walker, H.M., and J. Lev (1953)Statistical Inference. Henry Holt and Co., New York.