Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 9  |  Issue : 2  |  Page : 99-105

Tooth morphometry and the pattern of palatal rugae among monozygotic and dizygotic twins in India


1 Department of Oral Pathology and Microbiology, Institute of Dental Sciences, Siksha ‘O’ Anusandhan University, Bhubaneswar, Odisha, India
2 District Head Quarter, Bhadrak, Odisha, India
3 Department of Community Dentistry, Indira Gandhi Institute of Dental Sciences, Kothamangalam, Kerala, India

Date of Web Publication8-Jan-2018

Correspondence Address:
Dr. Swagatika Panda
Department of Oral Pathology and Microbiology, Institute of Dental Sciences, Siksha ‘O’ Anusandhan University, Bhubaneswar 751030, Odisha
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jofs.jofs_46_17

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  Abstract 


Introduction: A stronger correlation of physical traits among monozygotic (MZ) twins than dizygotic (DZ) twins indicates predominant contribution by genetic factors than environmental factors. Exploring the degree of resemblance in tooth morphometry and the pattern of rugae among twins shall aid in forensic identification. Aim and Objective: To find out the degree of correlation in tooth morphometry and the palatal rugae pattern among MZ and DZ twins. Materials and Methods: The maximum coronal mesiodistal and buccolingual dimensions of the maxillary teeth excluding the second and third molars in 21 pairs of MZ and 12 pairs of DZ twins along with a MZ triplet were recorded using digital calipers calibrated to 0.01 mm. The dimensions of the teeth based on the number, shape, size, and the unification palatal rugae pattern among twin pairs were analyzed and recorded. Results: Our results suggest a stronger correlation of tooth dimension among MZ than DZ twins, which differs for individual maxillary tooth. There may be a separate set of genes responsible for controlling the mesiodistal and buccolingual tooth dimensions. The maxillary canine and maxillary premolars do show the least amount of genetic variability. The results in our study provide remarkable evidence regarding the existence of mirror imaging in tooth dimension as well as the number and shape of the palatal rugae. Conclusion: This first of its kind study in the Indian population suggests a remarkable similarity with regard to the tooth’s size and uniqueness of the palatal rugae pattern among MZ and DZ twins, which suggests their strong inheritability potential. This may be useful as additional tools for zygosity determination along with other dental traits. The significant evidence of mirror imaging of tooth dimension and rugae shall definitely contribute to the concept of development of human body.

Keywords: Mirror imaging, morphometry, palatal rugae, twin, zygosity


How to cite this article:
Panda S, Sahoo A, Mohanty N, Sahoo SR, Subramaniam R. Tooth morphometry and the pattern of palatal rugae among monozygotic and dizygotic twins in India. J Orofac Sci 2017;9:99-105

How to cite this URL:
Panda S, Sahoo A, Mohanty N, Sahoo SR, Subramaniam R. Tooth morphometry and the pattern of palatal rugae among monozygotic and dizygotic twins in India. J Orofac Sci [serial online] 2017 [cited 2019 Jun 18];9:99-105. Available from: http://www.jofs.in/text.asp?2017/9/2/99/222389




  Introduction Top


Monozygotic (MZ) twins, otherwise known as identical twins, originate from the same fertilized ovum, by the virtue of which they share their amniotic sac and deoxyribonucleic acid (DNA). The more common dizygotic (DZ) twins are formed when two sperms fertilize two eggs separately, and they do not share the same DNA. Differences between MZ twins are usually of similar magnitude, apart from the minor left–right differences often noted in singletons. On the other hand, the DZ twins are expected to show almost similar or lesser intrapair variability as siblings.[1] The relative contributions of genetic and environmental influences can be studied by comparing physical features within twin pairs.

The size and shape of the tooth, referred as tooth morphometry, is determined at the morphodifferentiation stage of the tooth development, which is approximately at the 18th week of embryonic development for permanent teeth. Tooth morphometry is largely controlled by hereditary forces, and the pattern of heritability is distinct for each group of tooth.[2] It has been suggested that normal variation in the size of the tooth is the result of multifactorial inheritance, with both genetic and environmental factors being important.[3]

The palatal rugae, also called plicae palatinae transversae and rugae palatina, refer to the ridges on the anterior part of the palatal mucosa on each side of the median palatal raphe and behind the incisive papilla. At the 550-mm stage of embryonic development, there are five to seven rather symmetrically disposed ridges, with the anterior ones beginning at the raphe while the others move laterally. Toward the end of intrauterine life, the pattern of the rugae becomes less regular, with the posterior ones disappearing, and those that are anterior becoming considerably more pronounced and compressed.[4] The palatine rugae are permanent and unique to each person and can help to establish identity.[5]

Genetic, epigenetic, and environmental factors together influence both the pattern of palatal rugae and tooth morphometry. The influence of these factors on the pattern of palatal rugae and tooth morphometry may be studied by various means such as animal studies, twin studies, family studies, and genetic typing. Sir Francis Galton[6] was the first person to suggest that studies on twins would be particularly useful in defining the role of hereditary and environmental influences in determining the form and size of the human body. He suggested that this could be accomplished by comparing differences within the MZ twin pairs and DZ twin pairs. Following this study, many studies have been conducted globally producing inconsistent results. Therefore, we have conducted a study with the objective of evaluating the predominant influencing factors on tooth morphometry and palatal rugae patterns as well as to establish the role of tooth dimension and pattern of rugae in personal identification especially in crimes involving either or both of the twins.


  Materials and Methods Top


Sample size and zygosity determination

Ethical approval for this study (ethical committee number 210-5/16/01/2015) was provided by the ethical committee of Siksha ‘O’ Anusandhan University (deemed to be) dated 16.01.2015. The study sample consisted of 21 pairs of MZ and 12 pairs of DZ twins along with a MZ triplet. All the participants were in the age group of 8–64 years. The representative images of MZ and DZ twins are shown in [Figure 1] and [Figure 2], respectively. Zygosity was determined on the basis of physical appearance and a thorough history of the time of birth. Quick setting stone casts were prepared from the alginate (Zelgan Plus, Dentsply, Gurgaon, India) impressions of the maxillary teeth of each participant. Participants without any kind of intraoral appliances and developmental anomalies of the lip and palate were included in this study. This study was conducted after obtaining clearance from the institutional ethical committee and informed written consent from all the participants.
Figure 1: Similar degree of expression of the cusp of Carabelli in MZ twins

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Figure 2: Similar degree of expression of the cusp of Carabelli in DZ twins

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Tooth morphometry

Under the same uniform external illumination with the dental casts placed on a flat surface, mesiodistal and buccolingual dimensions of the maxillary teeth, except the second and third molars, were measured by the means of a digital caliper (Mitutoyo, Japan), which permitted readings to the nearest 0.01 mm. The sharpened beaks of the caliper were placed at the contact points and the instrument held parallel to the incisal/occlusal surfaces of the teeth, as the individual tooth was measured. The mesiodistal dimension was recorded as the maximum mesiodistal dimension of the tooth crown parallel to the occlusal and labial surfaces, and the buccolingual dimension was recorded as the maximum crown dimension at right angles to the mesiodistal dimension. Each tooth was measured on two separate occasions, and the mean of the measurements was used to improve accuracy. A discrepancy of more than 0.4 mm resulted in remeasuring the tooth. The carious and proximally restored teeth were excluded from the study.

Rugoscopy

The rugae patterns in the casts were analyzed with reference to the classification suggested by Thomas et al.[7] based on their length, shape, and unification. The rugae were marked on the casts using a black HB pencil. The number and shape of the rugae were assessed. Few shapes other than the conventional rugae patterns were also noticed, which were categorized into special type. All the parameters from each dental cast of each pair were observed and documented in the format.

Other dental features

Spacing, missing tooth, the time of eruption, and the expression of the cusp of Carabelli were carefully examined in all MZ and DZ dental casts.

Before data analysis, intraobserver agreement was evaluated by Kappa statistics to ensure that there was no significant measurement error. A total of 20 casts were abruptly selected, and tooth dimension as well as rugae morphology recorded again. Intraobserver agreement values for all teeth ranged from 0.885 to 0.926. Intraobserver agreement values for the shape and size of the palatal rugae ranged from 0.906 to 0.957.


  Results Top


Tooth morphometry

Demographic details

The final study sample consisted of 21 pairs of MZ (12 females and 10 males) and 12 pairs of DZ twins (three females, five males, and four opposite sexed) along with a MZ triplet. All twin pairs were selected from an Indian population.

Pearson’s correlation was performed using the Statistical Package for the Social Sciences version 21 software (IBM SPSS Statistics for Windows, Version 21.0, Armonk, NY: IBM Corp.). The correlation coefficients of statistical significance have been tabulated in [Table 1]. This is equivalent to a comparison of variances when the expected mean difference between twin pairs is zero. An F-test was used to determine significance.
Table 1: Statistically significant correlation coefficients of tooth morphometry among twins

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MZ twins were analyzed separately for males and females due to the morphometric differences based on gender.

The analysis of buccolingual dimension in MZ females showed maximum intrapair resemblance in the maxillary left incisor (0.853), left canine (0.685), right first premolar (0.875), and both left (0.859) and right second premolars (0.804). The mirror image effect was seen with both the incisors (0.720, 0.907) and second premolars (0.789, 0.807). The maximum intrapair resemblance in mesiodistal dimension was observed in the maxillary lateral incisors (0.940), maxillary canine (0.945), maxillary premolars (0.885, 0.943), and right maxillary first molar (0.720). Mirror image effect in relation to mesiodistal dimension was seen in the maxillary lateral incisor (0.637), canine (0.862, 0.908), second premolar (0.607, 0.705), and first molar (0.799,0.716) in MZ females.

The analysis of MZ males showed maximum resemblance in the buccolingual dimension of the maxillary right central incisor (1.000), both the lateral incisors (0.996, 0.965), and the right first premolar (0.882). The mirror imaging of the buccolingual dimension was seen in the lateral incisor (0.996), as well as the first (0.928) and second premolars (0.891).

Mesiodistal dimension analysis in MZ male twins showed the maximum resemblance between the right lateral incisor (0.955) and second premolar (0.947), whereas mirror image effect was seen in the maxillary central incisor (0.998), lateral incisor (0.962), first premolar (0.938), and second premolar (0.891, 0.891).

Analysis in DZ twin pairs yielded a strong intrapair buccolingual dimension resemblance of the right lateral incisors (0.748) and a mesiodistal dimension resemblance of the right central (0.695), lateral incisors (0.862), and second premolars (0.830). There was mirror image effect with respect to the maxillary central (0.719) and lateral incisor (0.596, 0.852) as well as in the maxillary first molar (0.759) with respect to the mesiodistal dimension of teeth among DZ twins. With respect to buccolingual dimension, mirror imaging was seen in the maxillary lateral incisor (0.814, 0.753).

Rugoscopy

Demographic details

The final study sample consisted of 21 pairs of MZ (12 females and 10 males) and 12 pairs of DZ twins (three females, five males, and four opposite sexed) along with a MZ triplet. All twin pairs were selected from an Indian population.

The right and left rugae patterns with respect to their number, length, shape, and unification between one twin and its co-twin in MZ pairs and between the twins of a DZ pair were compared [Table 2].
Table 2: Statistically significant correlation coefficients of the palatal rugae among twins

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Pearson’s correlation coefficient for the shape of primary rugae in twin pairs indicated the resemblance of rugae on the one side of one twin with the opposite side of its co-twin, both in MZ (0.502) and DZ twins (0.657). The correlation analysis of a number of rugae has shown us a strong correlation of one side of one twin and the opposite side of its co-twin, both in MZ (0.491) and DZ twins (0.609). This explains the existence of mirror imaging in the number and shape of palatal rugae.

Other dental features

The cusp of Carabelli was observed in 14 out of 17 MZ twins [Figure 1], whereas its presence was observed in six out of 13 DZ twins [Figure 2]. There was very less intrapair variance in the degree of expression in both MZ and DZ twins. Spacing, to a great extent, was similar in MZ twins [Figure 3], whereas it was very different in DZ twins. There was an interesting finding with regard to missing tooth in one of the MZ twins, in whom there was missing first premolar among co-twins in mirror image location [Figure 4]. Neither among intrapair MZ twins nor in DZ twins did the time of tooth eruption resemble.
Figure 3: Similarity in spacing among MZ triplets

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Figure 4: The missing first premolar among co-twins (MZ) in mirror image location

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  Discussion Top


Determining the contributions of epigenetic,[8] genetic, and environmental[9],[10] factors toward the dental features is the basis of studying twins and their relatives. Hughes et al. attributed 59–62% of the crown size to additive genetic variation, with environmental effects ranging from 8 to 29%.[11]

The study of tooth morphometry is directly relevant to understanding the causes of dental crowding and spacing. It is generally accepted that common dental anomalies, such as supernumerary teeth and oligodontia, are linked to tooth size and influenced by a common set of genes. Although we can expect a greater correlation of tooth dimension among MZ twins than DZ twins, it becomes a fundamental interest to establish the range of variation of tooth dimensions biometrically. Similarly, although there are many studies on fingerprints, there are not enough studies to evaluate the uniqueness of the palatal rugae among twins. A thorough PubMed search has indicated that this is the first of its kind study globally, which has tried establishing the resemblance of both tooth morphometry and palatal rugae among MZ and DZ twins to investigate the uniqueness and influence of the inheritance of these dental traits. The findings of this study shall contribute to the field of clinical dentistry, human biology, physical anthropology, and personal identification in forensic odontology.

Among a handful of studies on tooth morphometry, the first study was based on a sample of 58 pairs of Indian twins, which invalidates conventional genetic variance estimates and reveals considerable hidden environmental determinants.[12] Contradicting this concept, Townsend et al.[13] have indicated that tooth morphometry is under different control mechanisms, with environmental influences on buccolingual dimensions exceeding those on mesiodistal dimensions. This has been supported by our study stating that there is a significant contribution of genetic factors toward tooth morphometry, although there exists a different control mechanism for mesiodistal and buccolingual dimensions. The degree of correlation of tooth morphometry between the pairs of twins provides a measure of degree to which MZ and DZ twins are alike. The more the correlation, the greater is the contribution of genetic factor and least is the contribution of the environment. Our study has shown that there was a greater correlation of mesiodistal and buccolingual dimensions among MZ twins than DZ twins. The different results in MZ males and females indicate the effect of sexual dimorphism. Kabban’s study[1] indicated a greater correlation of the upper central incisor for the mesiodistal dimension, and the upper and lower first molars for the buccolingual dimension. The difference in opinion may be due to the exclusion of sexual dimorphism in Kabban’s study,[1] although the population-specific differences cannot be ignored. The maxillary lateral incisor in our study showed maximum correlation among MZ male and female twins except in the buccolingual dimension of female MZ twins, which is supported by Kabban’s study.[1] Rather than a common set of teeth, there are individual tooth that hold the least genetic variability for MZ male and female twins and DZ twins. The limited genetic variability of the canine in female MZ twins as observed in our study is comparable with Dahlberg’s hypothesis and Horowitz et al.’s study[9] of a comparatively slow rate of evolutionary change in the canine. Dahlberg[14],[15] in his adaptation of Butler’s[16] field concept of the human dentition considered the canine as morphologically stable, as concerns the expression and retention of ancestral patterns. The heritability of the canine is comparable to the premolars with the exception of the second premolar in the buccolingual dimension of male MZ twins and DZ twins. The analysis of tooth morphometry in our study indicated that the greatest effect of genetic factors is toward the dimension of the right maxillary second premolar, followed by the right maxillary first premolar. The heritability of the premolar is not yet studied, and this research represents a first of its kind. By comparison with the canine, the lateral incisor demonstrates little more genetic variability, which is comparable to Lundstrom’s twin study.[16]

Sex and other environmental components dictate the nongenetic variability, and it is apparent from our data that these factors accumulate particularly in the maxillary left and right incisors and the canine resulting in statistically significant sex differences in these teeth. With regard to bilateral asymmetry in male and female MZ twins in our data, we may suggest that nongenetic/environmental variability may be sex influenced. This also suggests that there may be some generalized sex-influenced asymmetry mechanism operating at the very primitive stage of tooth development. Significant sex differences in the canine tooth found in our study has also been observed in recent population studies.[17],[18]

The twinning process may be linked to a fascinating phenomenon known as mirror imaging, in which one member of a twin pair “mirrors” the other for one or more features. It has been reported that around 25% of MZ twins display mirror imaging.[19] Mirror imaging is typically limited to the ectodermal tissues such as the tooth. The size of the tooth in MZ twins is considered to be a mirror image when the difference in tooth dimensions between the left side of one twin and the right side of the other twin (L1–R2) is less than the same side difference between them (L1–L2) and (R1–R2) and vice versa.[20] Potter and Nance’s study[21] on dental dimensions and mirror imaging as well as Kabban’s[1] study found no indication that MZ twinning was associated with mirror imagery, whereas our study indicated that there was a significant mirror imaging in both MZ and DZ twins, although to a greater extent in MZ twins. This study presents a significant evidence of mirror imagery in the number and shape of the palatal rugae among both the MZ and DZ twins. The missing premolar in one of the MZ twins in mirror location in our result is also evidence for the mirror image concept.

The accurate determination of zygosity and chorionicity is essential in multiple circumstances. Although DNA analysis gives the ultimate evidence of zygosity, still the evidence of a strong significance of a high degree of tooth-specific correlation and mirror imaging of a certain tooth as seen in our result indicates that those particular tooth dimensions may help determining zygosity. Kabban[1] did not find a significant correlation of tooth morphometry; therefore, he had discouraged the use of the tooth to determine zygosity. A small number of comparable teeth might have been the reason of this. Lundstrom[16] suggested the use of occlusal morphology in determining twin zygosity.

However, one must also consider that studies have suggested that DZ twins may be more alike than siblings, because, unless reared apart, they share the same pre- and postnatal environment. Therefore, the difference in tooth size between DZ co-twins may also be less than expected. This has also been suggested by Sharma et al.[12] The findings of this study may add to our current knowledge regarding the environmental influence on human dentition and palatal rugae. In addition, this study may also help in indicating the effect of sexual dimorphism on dentition.Other dental traits such as spacing, crowding, and the angulation of the teeth are more or less similar to a large extent in MZ twins, whereas they may not even be comparable in DZ twins, which indicates that there exists a strong inheritance component of these dental traits. Our study also suggests a very low level of hereditary variability of the Cusp of Carabelli. This result supports Biggerstaff’s[22] and Laatikainen and Ranta’s study.[23]


  Conclusion Top


Innovative noninvasive approaches such as 2D image analysis and 3D laser scanning techniques, micro-CT equipment of the dental tissues, and the individual tooth from twin pairs may be implemented for the accurate identification of the quantity and the areas of resemblances. These approaches open up exciting new opportunities for clarifying how genetic, epigenetic, and environmental factors contribute to the variation of different dental tissues with different embryological origins. Studies comprising a very large sample size of twins should be encouraged, so that the observations would impact the process of forensic identification significantly.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Kabban M, Fearne J, Jovanovski V, Zou L. Tooth size and morphology in twins. Int J Paediatr Dent 2001;11:333-9.  Back to cited text no. 1
    
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Townsend G, Harris EF, Lesot H, Clauss F, Brook A. Morphogenetic fields within the human dentition: A new, clinically relevant synthesis of an old concept. Arch Oral Biol 2009; 54(Suppl 1): S34-44.  Back to cited text no. 2
    
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Pagni L, Baccetti T. [Heredity and environment in the genesis, epigenesis and evolution of the orofacial area]. Minerva Stomatol 1993;42:1-13.  Back to cited text no. 3
    
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Rajan VP, John JB, Stalin A, Priya G, Abuthagir AK. Morphology of palatal rugae patterns among 5–15 years old children. J Pharm Bioallied Sci 2013;5(Suppl 1):S43-7.  Back to cited text no. 4
    
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English WR, Robison SF, Summitt JB, Oesterle LJ, Brannon RB, Morlang WM. Individuality of human palatal rugae. J Forensic Sci 1988;33:718-26.  Back to cited text no. 5
    
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Galton F. The history of twins, as a criterion of the relative powers of nature and nurture (1, 2). Int J Epidemiol 2012;41:905-11.  Back to cited text no. 6
    
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Thomas CJ, Kotze TJ, Nash JM. The palatal ruga pattern in possible paternity determination. J Forensic Sci 1986;31:288-92.  Back to cited text no. 7
    
8.
Townsend GC, Richards L, Hughes T, Pinkerton S, Schwerdt W. Epigenetic influences may explain dental differences in monozygotic twin pairs. Aust Dent J 2005;50:95-100.  Back to cited text no. 8
    
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Horowitz SL, Osborne RH, DeGeorge FV. Hereditary factors in tooth dimensions, a study of the anterior teeth of twins. Angle Orthod 1958;28:87-93.  Back to cited text no. 9
    
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Osbourne RH, De George FV. Genetic Basis of Morphological Variation: An Evaluation and Application of the Twin Study Method. Cambridge, MA: Harvard University Press; 1959.  Back to cited text no. 10
    
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Hughes T, Townsend G, Bockmann M. An Overview of Dental Genetics. A Companion to Dental Anthropology. John Wiley & Sons, Inc; 2015. p. 121-41.  Back to cited text no. 11
    
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Sharma K, Corruccini RS, Henderson AM. Genetic variance in dental dimensions of Punjabi twins. J Dent Res 1985;64:1389-91.  Back to cited text no. 12
    
13.
Townsend GC, Richards LC, Brown T, Burgess VB. Twin zygosity determination on the basis of dental morphology. J Forensic Odontostomatol 1988;6:1-15.  Back to cited text no. 13
    
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Dahlberg AA. The changing dentition of man. J Am Dent Assoc 1945;32:676-90.  Back to cited text no. 14
    
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Dahlberg AA. Analysis of the American Indian dentition. In: Dental Anthroplogy. Elsevier; 1963.  Back to cited text no. 15
    
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Lundstrom A. Tooth morphology as a basis for distinguishing monozygotic and dizygotic twins. Am J Hum Genet 1963;15:34-43.  Back to cited text no. 16
    
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Khangura RK, Sircar K, Singh S, Rastogi V. Sex determination using mesiodistal dimension of permanent maxillary incisors and canines. J Forensic Dent Sci 2011;3:81-5.  Back to cited text no. 17
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Yuwanati M, Karia A, Yuwanati M. Canine tooth dimorphism: An adjunct for establishing sex identity. J Forensic Dent Sci 2012;4:80-3.  Back to cited text no. 18
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Springer SP, Deutsch G. Left Brain, Right Brain: Perspectives From Cognitive Neuroscience. New York, NY: W.H. Freeman; 2003.  Back to cited text no. 19
    
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Pelsmaekers B, Loos R, Carels C, Derom C, Vlietinck R. The genetic contribution to dental maturation. J Dent Res 1997;76:1337-40.  Back to cited text no. 20
    
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Potter RH, Nance WE. A twin study of dental dimension. I. Discordance, asymmetry, and mirror imagery. Am J Phys Anthropol 1976;44:391-5.  Back to cited text no. 21
    
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Biggerstaff RH. Heritability of the Carabelli cusp in twins. J Dent Res 1973;52:40-4.  Back to cited text no. 22
    
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Laatikainen T, Ranta R. Occurrence of the Carabelli trait in twins discordant or concordant for cleft lip and/or palate. Acta Odontol Scand 1996;54:365-8.  Back to cited text no. 23
    


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