Cusp Size, Sexual Dimorphism, and Heritability of Cusp Size in Twins ROBERT H. BIGGERSTAFF University of Kentucky, College of Dentistry, Department of Orthodontics, Lexington, Kentucky 40506

K E Y WORDS Twins.

Cusp size

.

Mandibular molars

.

Heritability

ABSTRACT Overall measures of mandibular molars reflect the combined size contributions of the component cusps and ridges. Until now, the size hierarchy of primary and permanent mandibular molar cusps remained unclear. This paper utilizes the relative plane surface areas (basal area dimensions) of the individual molar cusps, as assays of cusp size to demonstrate cusp size variations within populations, antimere cuspal variations, sexual dimorphism, and, the heritability of cusp size. Duplicate dental casts from 199 pairs of like-sexed twins provide the raw data. Defined anatomic landmarks on the occlusal surfaces were reduced to X-Y rectangular coordinates prior to the computation of the basal areas dimensions. The results establish a cusp size hierarchy specific for molar type, i.e., fivecusped molars with a distal fovea and distal marginal ridge (5fd), five-cusped molars without a distal fovea and without a distal marginal ridge (So), and fourcusped molars (4c). Sexual dimorphism in cusp size is apparent in 5fd molar cusps but not in 50 molar cusps. However, males have a significantly higher frequency of 5fd molars. Females have a higher frequency of smaller 50 and 4c molars which have fewer crown components. Moreover, female 50 molars have cusps as large as or larger than 50 male molar cusps. Right-side-left-side differences exist between antimere cusps based on relatively low correlations. The mirroring of molar types occurs infrequently. When observed, most intrapair differences for cusp size, using F-ratios, indicate a low component of hereditary variability.

Overall molar measurements reflect the combined size contributions of the component cusps and ridges. These, in turn, indicate the magnitude of soft and hard tissue growth during the processes of tooth maturation. Tooth size differences exist within and between populations and they appear to be continuously distributed. Presumably, then, there are cusp size variations. Yet, most quantitative dentition studies rely solely on mesiodistal and buccolingual crown measurements to establish phylogenetic and taxonomic trends among the major hominid populations. Dental anthropologists generally classify mandibular molars on the basis of groove pattern. However, the dryopithecus pattern (Gregory, '16; Gregory and Hellmann, '27) and its variations (Jsrgensen, '55) may not be the most accurate or consistent method (Biggerstaff, '68; Morris, '70). An alternaAM. J. PHYS. ANTHROP.,42: 127-140.

tive classification scheme ignores groove pattern and categorizes mandibular molars according to the number of crown components: fivecusped molars without a distal marginal ridge and fovea, 50; five-cusped molars with a distal marginal ridge and fovea, 5fd; and, four-cusped molars, 4c (Biggerst aff, '68). The relative size of each cusp is another source of disagreement. For example, Wheeler ('50) states that the mesiobuccal, distobuccal and distal cusps, are of equal size in the mandibular second primary molars. He further declares that the distal cusp of the mandibular first permanent molar is smaller than the other buccal cusps and that the mesiobuccal cusp is larger by a small margin than the two lin1 This research was supported in part by a National Institutes of Health Reserch grant DE-02506-01and by a General Research Grant to the University of Kentucky.

127

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ROBERT H. BIGGERSTAFF

gual cusps which are equal in size. Kraus and Jordan (‘65) consider the mesiolingual and distolingual cusps of mandibular first permanent molars to be of equal size, each larger than the three buccal cusps. Jorgensen (‘56) notes that the mesiobuccal cusp is the largest of the five cusps, whereas Zeisz and Nuckolls (’49) suggest that the distobuccal cusp is the largest cusp. Thus, it appears that the problem of cusp size hierarchy for the primary second and permanent first mandibular molars remains unresolved. Presumably tooth size, and by extension cusp size, has a high degree of heritability. The basis of this assumption is that teeth are good genetic indicators (Lasker, ’50; Kraus, ’57, ’62; Kraus et al., ’59; Osborne et al., ’58; Osborne and De George, ’59; Moorrees, ’62; Osborne, ’62, ’63; and Krogman, ’67). Unfortunately most studies of dental trait genetics are inconclusive. It is apparent that cusp size and cusp number are interacting variables which collectively reflect individual overall tooth size differences, sexual dimorphism, and within and/ or between population variations for these traits. The purpose of this study is fourfold: (1) to describe the range of size variation for each cusp according to defined molar types; (2) to demonstrate the cusp size hierarchy for mandibular molars; (3) to describe the magnitude of sexual dimorphism in cusp size; and, (4) to measure the degree of hereditary variability of molar cusp size using the co-twin method. METHODS A N D MATERIALS

The data for these studies were derived from duplicates of maxillary and mandibular dental casts secured from one hundred pairs of monozygotic twins (50 female and 50 male) and 99 pairs of dizygotic twins (49 female and 50 male). The twin pairs were identified initially on the basis of their history. Zygosity was ascertained primarily from blood group and serological studies. The tests were done by trained technicians using agglutination reactions to certified anti-sera to determine blood group comparability among the twin pairs. In addition, the sera were tested for haptoglobin and transferrin variants to detect intrapair comparability. Any twin pair failing one of

27 tests was classified as dizygotic (see Biggerstaff, ’69c for a more detailed discussion on zygosity determination). The tests were performed on all members of the family on the same day to minimize errors due to different test conditions (reagents, temperatur e variation s, and technicians) . The original casts were selected for duplication from an ongoing semilongitudinal twin study at the Forsyth Dental Center, Boston, Massachusetts (Biggerstaff, ’69c) and were made available to the writer by Dr. C. A. F. Moorrees. The selection was based on a low DMF index and the quality of the casts. The selected casts were duplicated using the reversible hydrocolloid technique paying careful attention to procedural detail. My previous reported experiences (Biggerstaff, ’69b) and those of Phillips and It0 (’51) indicate that these procedures consistently produce accurate duplicate casts. The duplicate dental casts were processed according to the photogrammetric methods of Biggerstaff (’69c). The defined anatomic landmarks (Biggerstaff, ’69a) were registered as India ink points. The registration accuracy was reported earlier (Biggerstaff, ’69b,c). The interobserver error for landmarks used in this study ranged from 0 mm to 0.1 mm. The anatomic landmarks were recorded as X and Y coordinates on an Image Plane Digitizer with an auxillary data processor control console connected to an IBM 526 Summary card punch. To determine the accuracy of digitizing the anatomic landmarks, the photographic records of 25 maxillary and mandibular casts were selected at random. These photographic negatives varied in the number of teeth and each tooth varied in the number of observable landmarks. The technician ( S . M.) and the writer digitized the photographic negatives on two different occasions while working independently. Computer programs were written to determine “within operator” and “between operator” error. The between operator error averaged 0.083 mm f 0.075 s.d. The number of paired points equalled 2,486. The “within operator” error averaged - 0.041 mm -+ 0.038 s.d. (Biggerstaff, ’69c). Computer programs corrected variations in photographic image size, and position such that computed dimensions between

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CUSP SIZE IN TWINS

end-points, e.g., the central fossae of right and left molars, did not differ significantly from Helios dial caliper measurements (Biggerstaff, ’69b,c). Exposed and processed photographic negatives detail the India ink points and fiducial data. The negatives are digitized, i.e., the anatomic landmark data are converted into X-Y rectangular coordinate data and stored on punched Hollerith cards (see Biggerstaff, ’69a, ’69b for greater detail). The programmed computer uses the coordinate data to calculate the relative plane surface areas of cusps and ridges. The surface area data are then stored on magnetic tape for statistical analyses. The rationale supporting these methods is published elsewhere (Biggerstaff, ’69c) and need not be presented in detail here. However, a brief summary is indicated. A cusp may be considered a polyhedron whose basal area can be projected onto a plane surface. The projected “top view” is equal to the basal area and is, therefore, quantifiable. Using anatomic landmarks defined as small India ink points on the occlusal surfaces, the molar cusp boundaries can be outlined by connecting vectors. Most authors interpret the size of a cusp as the area of its basal. surface so that size in this connection is a two-dimensional concept (J~rgensen,’56). Also, the basal area does not change appreciably because of occlusal wear. These convenient concepts can be readily utilized by the above methods. Several analyses can be applied to the surface area data. Only antimere cusp comparisons in individuals, corresponding cusp, comparisons in twin pairs, and cross-twin comparisons are considered in this study (fig. 1). To demonstrate sexual dimorphism and cusp size hierarchy, the individuals of the twin pairs were arbitrarily designated Twin A and Twin B (fig. 1). Thus, the male and female MZ and DZ twin pairs formed eight separate populations, two right sides and two left sides for each twin pair. Pooling of data was contraindicated. To do so would have significantly reduced the variances, particularly in the MZ individuals. Right and left side cuspal areas were computed separately for the primary second and permanent first mandibular molars according to the three previously defined molar types. For each data set, the com-

RIGHT

LEFT

Fig. 1 Antimere

and co-twin comparisons. cusp area comparisons; cusp area comparisons; (Ar X Br) or (A1 X B1) where A = Twin A, B = Twin B, 1 = left, and r = right. . . . . . . . Crosstwin cusp area comparisons; (Ar X B1) or (A1 x Br). After BiggerstaE (’73).

_ . _ . Antimere - - - Corresponding

puter was programmed to calculate descriptive statistics. The printout describing sexual dimorphism and cusp size hierarchy contained 96 tables. Each antimere cusp comparison was subjected to the paired t-test to detect significant differences. For this report, space and econmics demanded the sampling and/or condensation of the tabular data. Since four-cusped primary second and permanent first mandibular molars occurred in a low frequency, they were excluded from the sampling process. The remaining data were sampled using a table of randomly assorted numbers. The selected permanent molar data were then paired; i.e., if a 50 right MZ table was selected, the corresponding left 50 MZ table was also selected. The data set was completed by matching all Twin A and Twin B 50 tables. The means 2 one standard deviation are condensed and presented as illustrations. Each figure shows the range of variation in cusp areas for four separate populations. The randomly selected primary dentition data were not right-left paired as above. Instead, the actual table selected was matched for the other twin of the pair. The data are presented in a condensed format. Correlation coefficients for antimere cuspal relationships were also computer calculated to describe the variation within the individual. The data presented correspond to the printout tables selected by the randomized number method. Other programs were used to compute correlation coefficients, intrapair variances, and F-ratios for corresponding cusp com-

130

ROBERT H. BIGGERSTAFF

parisons and cross-twin cusp comparisons 5. In both data sets, the distolingual cusp (fig. 1). Homologous or corresponding cusp is the largest of the five cusps. The hiercomparisons disclose the variations between archy for cusp size is equally obvious: the the right cusp of Twin A and the corre- distolingual cusp; the mesiolingual cusp; sponding right cusp of Twin B (or the left the mesiobuccal cusp; the distobuccal cusp; cusp of Twin A and the corresponding left and, the distal cusp. Sexual dimorphism is cusp of Twin B). This analysis imposes a readily demonstrable for the three larger further restriction on the data since the cusps but is less well defined for the distocorresponding tooth type must be identical buccal and distal cusps. within twin pairing. Congruency does not Both data sets suggest that females genalways exist in the corresponding teeth of erally have fewer 5fd first permanent momonozygotic twin pairs (Biggerstaff, '70). lars than males. The reverse trend is sugCross-twin cusp area comparisons ex- gested for 50 first permanent mandibular press the variation between the right cusp molars (compare N's of figs. 2, 3 with N's area of Twin A and the corresponding cusp of figs. 4, 5). The distribution is statistically area on the left tooth of Twin B (or con- significant at the 0.05 level of probability versely, the left cusp area of Twin A and (XY = 4.619, d.f. = 1). the corresponding cuspal area on the right Cusp size and sexual dimorphism in tooth of Twin B). Again this analysis immandibular second deciduous poses a restriction on the data because the molar antimeres cross-twin tooth types must be identical. Mirroring is a relatively rare feature in the The distolingual cusp is the largest in teeth of monozygotic andlor dizygotic twin 50 mandibular second deciduous molars pairs (Biggerstaff, '70). and the distal cusp is the smallest (fig. 6). In both the corresponding and cross-twin The mesiolingual, mesiobuccal, and distocusp size analyses, variation in the popula- buccal cusps generally are equal in size. tion size (N's) occurs because only those Sexual dimorphism is demonstrable to a twin pairs with identical tooth types are degree for all cusps except the mesiolingual counted. The problem of incongruency in cusp. antimeres and discordance in correspondThe distolingual cusp (fig. 7) is the larging and/or cross-twin comparisons for man- est in 5fd second deciduous molars. The dibular molar types (Biggerstaff, '70) and distal cusp is the smallest. The mesiolinfor the Carabelli cusp trait (Biggerstaff, gual is the second largest while the mesio'73) has been noted previously. buccal and distobuccal generally are equal in size. Sexual dimorphism is evident only RESULTS for the mesiolingual cusps. Cusp size and sexual dimorphism in Antimere cuspal area relationships mandibular first permanent molar antimeres The correlation coefficients for antimere The mean values 2 one standard de- cuspal surface areas in the 50 mandibular viation are presented (figs. 2-5). The ran- first permanent molars of MZ individuals domly selected and condensed data dem- are presented in table 1. A definite trend onstrate cusp size, the hierarchy of cusp for bilateral equality in cusp size exists for size and the degree of sexual dimorphism males (r = 0.886 - 0.651). A trend tofor 50 and 5fd permanent molars in eight ward an inequality in cusp size is evident separate molar populations. The right and in the female correlation coefficients (r = left 50 mesiolingual and distolingual cusps 0.471 - 0.068). The size equality of antimere cusps in (figs. 2, 3) are relatively equal in size. The remaining cusps exhibit a demonstrable 5fd mandibular first permanent molars for hierarchy in cusp size. While the male and MZ individuals is less well defined (table female data exhibit a degree of variability, 2). Only the mesiolingual antimere cusps sexual dimorphism is not clearly discernible are significantly correlated in males (r = 0.584), whereas only the distolingual antiin these cusps. The descriptive statistics for 5fd right mere cusps are significantly correlated in and left first permanent mandibular ,molars females (r = 0.773). The significant corare presented graphically in figures 4 and relations could have occurred by chance.

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132

ROBERT H. BIGGERSTAFF

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134

ROBERT H. BIGGERSTAFF TABLE 1

Correlation coefficients f o r cuspal plane surface area dimensions i n (50) mandibzclar first p e r n a n e n t molar nnttmeres of m a l e and f e m a l e m o n o zygotic t w i n s . Antimere cuspal comparisons are of t h e right and left cusps i n individuals w i t h similar molar types Plane surface area dimensions

M . B. cusp

D.B. c u s p D.c u s p

D.L.c u s p M.L. c u s p 1 P < 0.01.

Male N = 2 6 r

0.8861 0.651 1 0.6801 0.7821 0.821 1

Female N = 2 7 r

0.218 0.068 0.471 0.246 0.373

TABLE 2

Correlation coefficients f o r cuspal plane s i i f a c e area dimensions 5fd i n mnndibularfirst p e r m a n e n t molar antimeres of m a l e and f e m a l e monozygotic twins. Antimere cuspal comparisons are of right and left cusps i n individuals w i t h similar molar types Plane surface area dimensions

M . B. c u s p D. B. c u s p D. c u s p D. L.c u s p M . L.c u s p 1

Male N = 19

Female N = 1 0

r

r

0.556 0.439 0.507 0.240 0.5841

0.479 0.670 0.422 0.7731 - 0.081

are signficantly correlated at the 0.01 level of probability (probably by chance). The 5fd mandibular second deciduous molar antimere cusps of the male DZ individuals (table 4) exhibit a definite trend toward bilateral cusp size equality (r = 0.763 - 0.404). This trend is less strong among females (r = 0.607 - 0.199). The male distolingual and mesiolingual antimere cusps are significantly correlated at the 0.01 level of probability. Only the female distolingual antimere cusps are significantly different from zero.

Corresponding c u s p relationships in twin pairs The relationship of corresponding cuspal areas for permanent molars in twin pairs is presented in tables 5 and 6. The data are inconclusive becauseof the small N’s which, in turn, refiect the nature of analytical restrictions. Included are the molars of those twin pairs whose right and left teeth are of the same type. In both data sets, females tend to exhibit a stronger trend toward corresponding cusp equality. The cross-twin correlations generally exhibit similar levels of cusp size equality and smaller N’s. TABLE 4

P < 0.01. TABLE 3

Correlation coefficients f o r czispal plane s u f a c e area dimensions i n 50 mandibular second decidzious molar antimeres of m o l e and f e m a l e monozygotic twins. Antimere cuspal comparisons nre of t h e right and left cusps i n individuals w i t h similar molar types Plane surface area dimensions

M . B. cusp D. B. c u s p D.c u s p D. L.c u s p M.L.c u s p

Male N = 2 3 r

0.466 0.7251 0.357 0.6041 0.479

Correlation coefficients f o r cuspal plane surface area dimensions i n Sfd mandibular second decidtcous molar antimeres of dizygotic t w i n s . Antimere cuspal comparisons are of t h e right and left czisps i n individuals w i t h similar molar types ~~

Plane surface area dimensions

M . B. cusp D . B. c u s p D. c u s p D. L.c u s p M.L.c u s p

Female N = 16 r

0.584 0.169 0.191 0.180 0.628

1

P

Male N = 18 r

0.429 0.484 0.404 0.7631 0.589I

Female N = 17 r

0.493 0.199 0.300 0.6071 0.435

< 0.01. TABLE 5

~~

1

P < 0.01.

The 50 mandibular second deciduous molar antimere cuspal area correlation coefficients are presented in table 3 for the individuals of MZ male and female pairings. The males generally exhibit a slightly stronger trend toward bilateral cusp size equality (r = 0.725 - 0.357) than the females (r = 0.628 - 0.169). The male distobuccd and distolingual ’antimere cusps

Correlation coeflzcients f o r corresponding czcsp p l a n e surface area dimensions f o r 50 right m a n d i b ular first permanent molars of dizygotic m a l e nnd f e m a l e twin pairs Plane surface area dimensions

Male N = 13

Female N = 8

r

I

~~~

M . B. cusp D. B.c u s p D. c u s p D. L. c u s p M . L. c u s p

0.252 0.009 0.490 0.677 0.467

0.514 0.813 0.434 0.091 0.478

135

CUSP SIZE IN TWINS

TABLE 6

TABLE 8

Correlation coefficients for corresponding cuspal plane surface area dimensions of 5fd left mandibular first permanent molars in mortozygotic male and female twin pairs

Correlation cotfficients for corresponding cuspal plane sulface area dimensions for 50 right mandibular second deciduous molars of dizygotic male and female pairs

Plane surface area dimensions

Male N = 5 r

Female N = 4 r

Plane surface area dimensions

Male N = 4 r ~

M. B . cusp D.B. cusp D.cusp D.L.cusp M. L.cusp

- 0.388 0.242 - 0.640 0.639 0.469

0.566 -0.113 0.710 0.674 - 0.369

M. B. cusp D. B. cusp D. cusp D. L.cusp M. L.cusp P

1

These observations tend to confirm that the congruency of corresponding molars is not a consistent trait in twin pairs. The intrapair correlation coefficients of cross-twin (fig. 1) cuspal areas in 5fd (table 7) and 50 (table 8) mandibular second deciduous molars suggest that females have a definitive trend toward cusp size equality. However, the small N's make these data difficult to interpret. They again reflect the restrictions on the data and the paucity of twin pairs with left and right (or right and left) molars of the same type. Mirroring is not prevalent in mandibular second deciduous molars. Heritability of cusp size, co-twinmethod The intrapair variances for corresponding cusp sizes suggest a low degree of heritability in 50 mandibular first permanent molars (table 9). The MZ variances, as expected, are usually smaller than the DZ variances; however, the computed F-ratios are rarely significant. Occasionally, a MZ variance will exceed the DZ variance, e.g., the distal cusp in table 9. This problem also appears in the primary second molars (table 10) and the same cusp is involved. While unexplained, these instances do not detract from the general trend toward smaller

~~

0.688 0.820 0.882 0.974 1 0.478

< 0.01. TABLE 9

Intrapair variances and F-ratios for male monozygotic and dizygotic left mandibular first permanent molars ( s o ) Plane surface area dimensions

N

Variance

F-ratio

M. B. cusp MZ DZ

8 10

0.3628 0.5000

1.3781

1.1142 1.4199

1.2744

DZ

1.1826 0.5870

2.01461

D. L. CUSP MZ DZ

1.3761 1.5117

1.0985

M. L. cusp MZ DZ

0.9263 1.4301

1.5439

D. B. cusp

MZ DZ D. cusp

MZ

1

MZ variance greater than DZ.

MZ variances, a trend that is apparent throughout the data which compares the MZ-DZ variances of corresponding cusps in permanent and deciduous molars. DISCUSSION

TABLE 7 Correlation coefficients for corresponding cuspal plane surface area dimensions for Sfd left mandibular second deciduous molars of monozygotic male and female twin pairs Plane surface area dimensions

Male N = 8 r

Female N = 9 r

M. B . cusp D.B. cusp D. cusp D. L. cusp M. L. cusp

- 0.383

0.721 0.366 0.743 0.409 0.746

0.264 0.140 0.703 0.824

0.203 - 0.006 0.365 0.283 0.617

Female N = 4 r

These data demonstrate that a definite hierarchy of cusp size exists within the observed molars. The hierarchy observed in these data is at variance with published reports (Zeisz and Nuckoll, '49; Wheeler, '50; J~rgensen,'56; Kraus and Jordan, '65). More importantly, this matter is not a simple compare andlor contrast matter, for it appears to be partially dependent upon the number of cusps and may be sex-linked. For example, the distolingual cusp tends to be

136

ROBERT H . BIGGERSTAFF TABLE 10

Intrapair variances and F-ratios f o r male monozygotic and dizygotic right mandibular second p r i mary molars (50) ~~

Plane surface area dimensions

N

Variance

F-ratio

6 4

0.2271 1.1231

4.94321

D.B. cusp MZ DZ

0.3507 1.6790

4.7876

D.cusp MZ DZ

0.2114 0.1598

1.32291

D.L. cusp MZ DZ

0.1120 3.3113

29.56522

0.4114 0.4882

1.1867

M.B. CUSP MZ

DZ

M.L. cusp MZ DZ 1

MZ variance greater than DZ.

2

P

c

0.01.

one of the largest cusps among those observed. However, it is unquestionably dominant only in the 5fd molars, a molar type that occurs most frequently in males (Biggerstaff, ’69c). Sexual dimorphism is also apparent in many cusp size measurements. The quantitative differences between comparable cusps in males and females are small and, therefore, do not achieve significance at the 0.05 level of probability. But, in the 50 molars, there is a tendency for certain female cusps to equal or exceed the corresponding cusp measurements for males. Females generally have smaller molars because they have a higher frequency of 50 and 4c molars. But female 50 molars are as large as or larger than those of males. These, in turn, are smaller than 5fd molars because they contain a fewer number of crown components (Biggerstaff, ’69c). Therefore, the higher frequency and larger overall dimensions of male 5fd molars create a dimorphic trait when all tooth types are pooled and categorized by sex. Sexual dimorphism in overall measures of molar size, i.e., mesiodistal and buccolingual diameters, is well documented (Garn and Lewis, ’64; Garn et al., ’66). The source of these variations may be that each cusp and marginal ridge contributes to the over-

all size of the tooth. Fewer cusps and ridges produce smaller teeth , The recognition that each crown component represents the summation of soft and hard tissue growth during the development of the tooth is of great importance. The collective crown component sizes, therefore, determine crown size. Thus, mesiodistal and buccolingual measures of tooth size are comparable to length and breadth measures of skull size. The question arises relative to the biological relevance of such measures. They are composite measures, extending across a number of relatively independent but highly correlated grown units. Each crown component varies in its basal area andlor position because of the interaction between the inherent potential for growth, the influences of the contiguous structures, and the environment. Certain assays of overall tooth size suggest a “morphogenetic patterning effect.” Apparently this is a part of “a developmental patterning in the sense that teeth and bone must develop synchronously in order that functional dental relationships -occlusion - can be facilitated” (Krogman, ’67). The relatively high antimere cuspal correlations reflect this patterning effect. Moreover, the total occlusal surface areas of the posterior teeth must be profoundly affected by the patterning effect in individuals with good occlusion where right side-left side tooth size differences exist. Even where there are obvious differences in the sizes of antimere crown components, the mesiodistal and/or buccolingual crown diameters may be equal. These observations may explain the lack of measurable asymmetry in the mesiodistal diameters of mandibular first permanent molar antimeres (Garn et al., ’66b). The crown component variations, particularly between antimeres, suggest that there may be genetic factors operating in the dentition such that right-side-left-side differences could be causally independent. Teeth, like hair, nails and fingerprints, develop initially because of a complex epithelio-mesenchymal interaction (Kollar and Baird, ’69, ’70). Once initiated, it is the mesenchyme that is responsible for soft tissue tooth morphology and initial hard tissue morphology. Major crown components such as cusps and ridges are preformed in soft tissue due to the differential

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growth of mesenchyme. The mature molar phenotype is, therefore, a manifestation of the complex epithelio-mesenchymal interactions, differential growth of epithelium and mesenchyme, and hard tissue deposition. To assign cause and effect in tooth morphology is exceedingly difficult. There is, at the present, enough valid information to suggest that asymmetry in the dentition is a rule rather than the exception. Could it be that the genetic factors controlling the differential growth of mesenchyme on the left side may differ slightly from that on the right side? I suggest that the presence of a right 50 molar and a left 5fd (or 4c) molar is an expression of the variation in the epithelio-mesenchymal interaction. The genetico-environmental contributions are elusive. The variation in equality of antimere cusp size detects for the first time a possible source of dental asymmetry. It is possible that local environmental factors may infiuence the regional mesenchymal specificity causing variations in the size and/or presence of antimere crown components. Several investigators (Ballad, '44; Ballard and Wylie, '74; Bolton, '58; Garn et al., '66a) present evidence in their data for antimere discordance using mesiodistal diameter measurements as their primary criteria. The magnitude of this discordance may exceed 2.5 mm (Keene, '67). There are no comparative data relating crown component variability to mesiodistal diameter. The dogma of dental bilateral symmetry is well rooted in anthropology, and is often based on data derived from left side measurements with an occasional right tooth substituted for a missing or damaged left tooth. Occasionally, the averaged dimensions of antimeres are reported. The rationale supporting the above methods is based on the nonsigniticance of overall antimere measurement differences. The problem of molar type differences was heretofore ignored, as were the important contributions of the cusp and ridges to total tooth size. The reported antimere cuspal data suggest that the underlying developmental processes of each crown component leads to dental asymmetries. Moreover, those who study dental morphology must recognize these problems and carefully consider them when assigning cause and effect.

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The corresponding intrapair correlations and variance ratios suggest that molar cusp areas exhibit a "relatively low hereditary component of variability" (Horowitz et al., '58). The reported findings parallel the results presented for canines mentioned in the above report. The F-ratios indicate that the MZ-DZ cusp area variances generally do not differ significantly. The results in this paper are at variance with literature data (Holt, '57, '61; Horowitz et al., '58; Osborne et al., '58; Lundstrom, '67; Menezes et al., '74). On the other hand, my data agree with those of Aoyagi ('67), Biggerstaff ('70, '72) and Staley and Green ('71, '74). They also agree with Osborne's ('62) concordance rates for the torus palatinus, Spuhler's ('50) morphological trait data (superficial thoracic veins, peroneus muscle distribution, and vallate papillae distribution) and Ogawa's ('40) studies of the vallate papillae. Certain analyses of dental traits in twins, i.e., Carabelli cusp (Biggerstaff, '72), mandibular molar type asymmetry (Biggerstaff, '70) and molar and premolar cusp asymmetry (Staley and Green, '72, '74), are remarkably consistent. These observations suggest that the more detailed (smaller) the trait observed the greater the probability for asymmetry within the individual or between twin pairs. This concept is compatible with the literature data cited above. It is at variance with literature data based on overall trait measurements. Considering the composite nature of most overall measurements, there may be some questions raised relative to the hereditary variability of traits studied in this manner. The methods of this study are informative and establish a quantitative cusp size hierarchy for mandibular first permanent and second primary molars. Sexual dimorphism for cusp size is demonstrable for most cusps but the statistical analyses do not reveal significant differences. Females tend to have fewer crown components, hence smaller teeth when overall measures are employed. Antimere cusp size comparisons suggest that dental asymmetrys originate during the early developmental stages of the teeth. Sidedness in dental variations is the rule rather than the exception, especially in observations which do not use overall tooth size measurements. Intrapair measurements of molar cusp size in MZ

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and DZ twins indicate a relatively low hereditary component of variability which is consistent with several other trait studies in twins. LITERATURE CITED Aoyagi, F. 1967 Morphogenetical studies on the similarities in the teeth and dental occlusion of twins. I. Tooth size as shown in photographs of the occlusal plane. Shikwa Gakoho., 67: 1-49. Ballard, M. L. 1944 Asymmetry in tooth size: A factor in etiology, diagnosis and treatment of malocclusion. Angle Orthodont., 14: 67-70. Ballard, M. S ., and W. L. Wylie 1947 Mixed dentition case analysis-estimating size of unerupted permanent teeth. Am. J. Orthodont. and Oral Surg., 33: 754-759. Biggerstaff, R. H. 1968 On the groove configuration of mandibular molars: The unreliability of the “Dropithecus pattern” and a new method for classifying mandibular molars. Am. J. Phys. Anthrop., 29: 4411144. 1969a T h e basal area of posterior tooth crown components. Am. J. Phys. Anthrop., 31: 163-170. 1969b Electronic methods for the analysis of the human post-canine dentition. Am. J. Phys. Anthrop., 31 : 235-242. 1969c A quantitative and qualitative study of the post-canine dentition in twins. Unpublished dissertation. 1970 Morphological variations for mandibular first permanent molars in monozygotic and dizygotic twins. Archs. Oral Biol., 15: 721730. 1973 Heritability of the Carabelli cusp in twins. J. Dent. Res., 52: 40-44. Bolton, W. A. 1958 Disharmony in tooth size and its relation to the analysis and treatment of malocclusion. Angle Orthodont., 28: 113-130. Garn, S . M., and A. B. Lewis 1964 Sex difference in tooth size. J. Dent. Res., 43: 306. Garn, S. M., A. B. Lewis, A. A. Dahlberg and R. S. Kerewsky 1966 Interaction between relative molar size and relative cusp number. J. Dent. Res., 45: 1240. Garn, S. M., A. B. Lewis and R. S. Kerewsky 1966a Bilateral asymmetry and concordance in cusp number and crown morphology of the mandibular first molar. J. Dent. Res., 45: 1820. 1966b The meaning of bilateral asymmetry i n the permanent dentition. Angle Orthodont., 36: 55-62. Gregory, W. K. 1916 Studies on the evolution of the primates. Part I. T h e Cope-Osborn “theory of trituberculy” and the ancestral molar pattern of the primates. Part 11. Phylogeny of recent and extinct anthropoid, with reference to the origin of man. Bull. Amer. Mus. Nat. Hist., 35: 239355. Gregory, W. K., and M. Hellman 1927 The dentition of Dryopithecus and the origin of man. Anthrop. Papers Am. Mus.Nat. H i s t , 28: 1-122. Holt, S. B. 1957 Quantitative genetics of dermal ridge-patterns on finger. Acta Genetica et Statistics Medica (Basel), 6: 473-476. 1961 Quantitative genetics of fingerprint patterns. Brit. Med. Bull., 17: 247-250. Horowitz, S . L., R. H. Osborne, and F. V. DeGeorge 1958 Hereditary factors in tooth dimensions: a

study of the anterior teeth of twins. Angle Orthodont., 2 8 : 87-93. Jplrgensen, K. D. 1955 The Dryopithecus pattern in recent Danes and Dutchmen. J. Dent. Res., 34 : 195-208. 1956 The deciduous dentition. Acta Odont. Scand., 14 (Supple.) 20. Keene, H. J. 1967 Australopithecine dental dimensions in a contemporary population. Am. J. Phys. Anthrop., 27: 379-384. Kollar, E. N., and G. R. Baird 1969 Tissue interactions in embryonic mouse tooth germs. J. Dent. Res., 21: 131-148. 1970 Tissue interactions in embryonic mouse tooth germs. J. Embryol. Exper. Morph., 24: 159-171. Kraus, B. S. 1957 The genetics of the human dentition. J . Forensic Sciences, 2 : 420-428. 1962 Areas of research in dental genetics. In: Genetics and Dental Health. C. Witkop, ed. McGraw-Hill Co., New York. Kraus, B. S., and R. E. Jordan 1965 The Human Dentition Before Birth. Lea and Febiger, Philadelphia, 218 pp. Kraus, B. S., W. J. Wise and R. H. Frei 1959 Heredity and the craniofacial complex. Am. J. Orthodont., 45: 172-217. Krogman, W. M. 1967 The role of genetic factors in the human face, jaws, and teeth: A review. T h e Eugenics Review, 59: 165-192. Lasker, G. 1950 Genetical analysis of racial traits of the teeth. Cold Spring Harbor Symposia on Quantitative Biology, IS: 191-203. Lunstrom, A. 1963 Tooth morphology as a basis for distinguishing monozygotic and dizygotic twins. Am. J. Hum. Genet., 1 5 : 3 4 4 3 . Menezes, D. M., T. D. Foster and C. L. B. Lavelle 1974 Genetic influences on dentition and dental arch dimensions: A study of monozvgotic and dizygotic triplets. Am. J: Phys. Anth;op., 40: 213-220. Moorrees, C. A. F. 1962 Genetic considerations i n dental anthropology. In: Genetics and Dental Health. C. Witkop, ed. McGraw-Hill Co., New York. Morris, D. G. 1970 On deflecting wrinkles and the Dryopithecus pattern in human mandibular molars. Am. J. Phys. Anthrop., 32: 97-104. Ogowa, H. 1940 Studien uber die umwallten Zugenpapillen bei den japanischen Zwillingsfeten. Folia anat. Japon, 19: 377-390. Osborne, R. H. 1962 Application of twin studies to dental research. In: Genetics and Dental Health. C. Witkop, ed. McGraw-Hill Co., New York. 1963 Respective role of twin, sibling, family and population methods in dentistry and medicine. J. Dent. Res., 42: 1276-1287. Osborne, R. H., and F. V. DeGeorge 1959 Genetic Bases of Morphological Variation. Harvard University Press, Cambridge, 204 pp. Osborne, R. H., S. L. Horowitz and F. V. DeGeorge 1958 Genetic variation in tooth dimensions: A twin study of the permanent anterior teeth. Am. J. Hum. genet., 10: 350-356. Phillips, R. W., and B. Y. Ito 1951 Factors influencing the accuracy of reversible hydrocolloid impressions. J. Am. Dent. Ass., 43: 1-17. Spuhler, J. N. 1950 Genetics of three normal morphological variations: Patterns of superficial

CUSP SIZE IN TWINS veins of the anterior thorax, peroneus tertius muscle, and number of vallate papillae. Cold Spring Harbor Symposia on Quantitative BiolOgy, 15 : 175-1 89. Staley, R. N., and L. J. Green 1971 Bilateral asymmetry in tooth cusp occurrence in monozygotic twins, dizygotic twins, and non-twins. J. Dent. Res., 50: 83-89.

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___ 1974 Types of tooth cusp occurrence asymmetry in human monozygotic and dizygotic twins. Am. J. Phys. Anthrop., 40: 187-196. Wheeler, R. C. 1950 A Textbook of Dental Anatomy and Physiology. Second ed. W. B. Saunders Co., Philadelphia, 422 pp. Zeisz, R. C., and J. Nuckolls 1949 Dental Anatomy. C. V. Mosby Co., St. Louis, 486 pp.