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Discrete dental trait asymmetry in Mexican and Belizean groups.

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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.
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