The relative contribution of genes and the environment to the&nbsp;etiology of malocclusion has been a matter of controversy&nbsp;throughout the twentieth century and the first decace of twentyfirst&nbsp;century. Twin studies provide important insights into how&nbsp;genetic and environmental factors contribute to variation in&nbsp;dental and craniofacial morphology. This review describes&nbsp;research models involving twins, apart from the traditional&nbsp;comparison of similarities in monozygotic (identical) and&nbsp;dizygotic (nonidentical) pairs, throws some light on zygositydetermination, summarizes some landmark twin studies in&nbsp;orthodontics and future directions in dental research involving&nbsp;twins are outlined

Gupta, Bhaskar

Patel, Dolly P

Sharma, Tanmay

English

Atharva Scientific Publications (E-Jounals)

Dolly P Patel et al
48
JAYPEE
REVIEW ARTICLE
Twin Studies: Revealing the Genetic Basis of Malocclusion
Dolly P Patel, Bhaskar Gupta, Tanmay Sharma
ABSTRACT
The relative contribution of genes and the environment to the
etiology of malocclusion has been a matter of controversy
throughout the twentieth century and the first decace of twenty-
first century. Twin studies provide important insights into how
genetic and environmental factors contribute to variation in
dental and craniofacial morphology. This review describes
research models involving twins, apart from the traditional
comparison of similarities in monozygotic (identical) and
dizygotic (nonidentical) pairs, throws some light on zygosity
determination, summarizes some landmark twin studies in
orthodontics and future directions in dental research involving
twins are outlined.
Keywords: Twins, Twin studies, Heredity, Zygosity determination,
Monozygotic and Dizygotic.
How to cite this article: Patel DP, Gupta B, Sharma T. Twin
Studies: Revealing the Genetic Basis of Malocclusion. J Orofac
Res 2012;2(1):48-51.
Source of support: Nil
Conflict of interest: None declared
INTRODUCTION
The classical twin research design, involving comparisons
of similarities in monozygotic (MZ) or identical pairs and
dizygotic (DZ) or fraternal pairs, has enabled researchers
to quantify the relative contributions of genetic (nature) and
environmental (nurture) factors to variation in many human
physical and behavioral features and disorders. Indeed, there
have been several recent reviews emphasizing the value of
twin studies in clarifying how genetic factors affect common
dental problems, such as dental caries, periodontal diseases
and malocclusion.1 However, with recent advances in
molecular biology and the outcome of the Human Genome
Project, some may feel that twin studies have become less
important or have lost their relevance altogether. This paper
highlights research designs involving twins, other than the
classical approach, that have the potential to further our
understanding of the role of genes in malocclusion. The
complex problem of malocclusion is still the most common
reasons for people seeking dental treatment but their genetic
bases remain unclear.
Why to Study Twins?
Twin studies are one of a family of designs in population
genetics which aid the study of individual differences by
highlighting the role of environmental and genetic causes
on different traits. Twins are invaluable for studying these
important questions because they disentangle the sharing
of genes and environments.2 If we observe that children in
a family are more similar than might be expected by chance,
this may reflect shared environmental influences common
to members of family class, parenting styles, education, etc.
but they will also reflect shared genes, inherited from
parents.2
The twin design compares the similarity of monozygotic
or identical twins, who share nearly 100% of their genetic
polymorphisms, to that of dizygotic or fraternal twins, who
share only 50% of their polymorphisms. By studying twins,
researchers can then understand more about the role of
genetic effects and the effects of shared and unique
environment effects. In any trait, both gene and environment
are involved and both of them collectively contribute to the
ultimate phenotype and it is very difficult to make out their
respective contribution, however twin studies can do so by
comparing monozygotic with the dizygotic twins. This gives
us the information about the relative contribution of genes
and environment and how the two interact. Studying
monozygotic (identical twins) alone does not solve our
purpose as they have not only a similar genetic makeup but
also a similar environment. On the other hand dizygotic
(nonidentical twins) acts as a control, as they too share the
same early environmental factors, but are not genetically
alike. The researchers look for traits that show a greater
similarity in identical twins than in nonidentical twins—
this greater similarity must indicate a shared genetic basis
for the trait. This is expressed as a percentage known as
‘heritability’.
Zygosity Determination
Zygosity determination3 mainly aims toward investigating,
if the twins are dizygotic (arising from two zygotes) or
monozygotic (arising from a single zygote), comparisons
of physical features, for example body build, facial
appearance, eye color and ear form can provide a reasonably
accurate means of distinguishing between MZ and DZ twin
pairs, mistakes can still be made, another way is by the use
of a questionnaire method which yields an accuracy of about
94%, but this method also has its limitations.3 Following
the discovery of blood groups and enzyme polymorphisms,
it became possible to determine whether twins were
monozygotic or not with relatively high probability. More
recently with advances in DNA technology, a small number
of highly polymorphic DNA markers can be used to establish
zygosity with very high probability.3 Rather than obtaining
10.5005/jp-journals-10026-1011
Twin Studies: Revealing the Genetic Basis of Malocclusion
Journal of Orofacial Research, January-March 2012;2(1):48-51 49
JOFR
blood samples to extract DNA, it is now possible to isolate
DNA from cheek cells obtained using buccal swabs.
Different Types of Twin Designs
The MZ Co-twin Design2
A powerful approach to clarify the roles of genetic and
environmental influences on normal features, or
susceptibility to diseases, involves studying MZ twin pairs
who show different phenotypic expressions for a particular
trait or disease under investigation.2
Alternatively, researchers can manipulate the
environment, so that each member of a twin pair is exposed
to different environmental conditions. Monozygotic twin
pairs are matched perfectly for age and sex and, as they
share the same genes except in very rare circumstances, any
differences between them will normally reflect environmental
effects.
Twins Reared Apart2
The ongoing studies of Tom Bouchard et al in Minnesota
involve examination of MZ twin pairs who were separated
at birth and then reared apart in different surroundings. This
model overcomes one of the often mentioned limitations of
traditional twin studies – that MZ twin pairs may be more
similar than DZ pairs for particular features, not because
they share more genes, but because they share more of the
environmental factors that influence the trait in question.2
Opposite-sexed DZ Twins2
If zygosity and sex are considered, there are five main groups
of twin pairs that may be studied: MZ female pairs, MZ
male pairs, DZ female pairs, DZ male pairs and DZ male-
female pairs. Comparison of correlations between same-sex
DZ pairs and opposite-sexed DZ pairs provide an
opportunity to investigate whether primary or secondary
sexual characteristics are affected by diffusion of hormones
in utero.2
The Twin Half-sib Model2
This model involves making comparisons between the
offspring of MZ pairs and their partners. The advantage
of this model is that the children of MZ pairs who are
born to different mothers are themselves genetically half-
siblings.2
Because MZ twin pairs have identical genotypes, their
children will share, on average, half their genes whether
they have the same twin as a parent or not. Monozygotic
pairs, their spouses and their offspring share different levels
of genetic relationship, ranging from zero for the spouses
of twins in the absence of assortative mating, to unity for
the twins themselves.2
Some Landmark Studies in Orthodontics
The Pioneer
Galton in 18894 was the pioneer and first to discuss the merits
of twin investigations. Comparisons of the between twin
pair differences yielded relative assessments of the influence
of heredity and environment, if the twin pairs were raised
in relatively identical environments.
Growth and Development
Garn, Lewis and Poleacheek5studied dental developmental
stages for mandibular 1st and 2nd molars in two sets of
triplets and suggested that dental development is for the
most part genetically determined. Picacintini6 applied the
co-twin study method on six sets of same sex triplets in
whom the zygosity had previously been determined to a
high degree of probability. He concluded that combined
increment growth of maxilla and mandible of MZ co-twin
show significantly smaller intrapair differences as compared
to that of dizygotic co-twins. This proves the role of heredity
in governing the growth of these individual bones.
Craniofacial Complex
Kraus et al7 in a twin study found out almost perfect
concordance was found in the craniofacial complex in the
MZ triples whereas only a low degree of concordance was
found in DZ triplets. They concluded that the morphology
of all the bones of the craniofacial complex was under rigid
control of the hereditary factors. Horowitz, Osborne and
De George8 did a cephalometric study of craniofacial
variation in 56 twin pairs and concluded that there is a highly
significant genetic variation in anterior cranial base,
mandibular body length and lower face height. They also
stated that the upper face height is the more stable element
in the facial profiles as it does not contribute greatly to the
genetic variability of the face as a whole. Lower face height
demonstrates a large degree of hereditary variability.
Tooth Size/Occlusal Variations
Detlefsen9 stated that tooth shape and size and arch shape
and size, were largely determined by hereditary
constitutional factors. Lundstrom10 studied tooth size and
occlusion in twins. He concluded that heredity is an
important factor in malocclusion. In class I cases, he found
87.3% of MZ twins and 84.6% of DZ twins and in class II,
67.7% of MZ twins and 10% of DZ twins having
concordance. Horowitz et al11 carried out a study on 54 pairs
Dolly P Patel et al
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JAYPEE
of like sexed adult twins. They studied the tooth dimension
of maxillary and mandibular permanent anterior teeth and
found strong genetic component of variability of the four
maxillary and four mandibular incisor teeth. The canine teeth
demonstrated relatively low hereditary component of
variability. The differences between the monozygotic and
dizygotic twin pairs were statistically highly significant for
all the teeth. Corruccini and Potter12 studied occlusal
variation in sample of 32 MZ and 28 DZ twins pairs. Teeth
displacement and crossbite were the most significantly
heritable criteria of occlusion. Significant heritability of
overjet, buccal segment relation, overbite, tooth rotation/
displacement could not be documented. These results
demonstrated a considerably increased environmental
component of variance in occlusion.
Arch Shape and Palatal Morphology
Goldberg13 stated for the arch form, that with a few
exceptions, unilateral difference between twins were less
than bilateral differences between the arch sides of the same
twin. Shapiro et al14 in his study on 102 pairs of twins found
that there is greatest influence of genetic factors on palatal
height. For palatal length, greatest contributions from
environmental factors determined its variability. Riquelme
and Green 15studied palatal width, length and height in 32
pairs of like- sexed Caucasian twins. The palatal width and
height and length dimensions revealed a significant
component of hereditary variability. Thus, heredity is
suggested as strong etiological factor in malocclusions where
palatal dimensions are involved. No significant sex
differences were found to exist between any of the mean
intrapair variances of the MZ and DZ twins.
DISCUSSION
Skeletal jaw discrepancies and malocclusion of genetic
origin can be successfully treated orthodontically, except
in extreme cases where surgical intervention is required.
This is because it is possible to modify the direction of
dentofacial growth using orthodontic appliances and
therefore change or forestall morphogenetic abnormalities.
Orthodontic correction of a malocclusion is in effect altering
the phenotypic expression of a particular morphogenetic
pattern. The degree to which this can be successfully
achieved depends on: (1) The relative contribution of each
factor to the existing problem. (2) The extent to which
skeletal pattern can be influenced by orthodontic and
orthopedic appliances.
In clinical orthodontics it must be appreciated that each
malocclusion occupies its own distinctive slot in the genetic
/environmental spectrum and therefore, the diagnostic goal
is to determine the relative contribution of genetics and
the environment. The greater the genetic component, the
worse the prognosis for a successful outcome by means of
orthodontic intervention. The difficulty, of course, is that
it is seldom possible to determine the precise contribution
from hereditary and environmental factors in a particular
case.
In dentofacial structure and malocclusion are primarily
genetic, e.g. severe mandibular prognathism or endogenous
tongue thrust, the treatment will either be palliative or
surgical. The search for a solution would ultimately focus
on delineating the responsible genes. Conversely, if
components of dentofacial structure and malocclusion have
trivial heritabilities, then the search needs to be directed at
environmental factors including malocclusion during growth
and development. The dentition in particular provides a very
useful model system to investigate developmental
mechanisms, given that teeth begin to develop soon after
conception and then form in an orderly sequence over an
extended period of time. Once formed, teeth are not re-
modelled, so they can be used to make retrospective
assessment of how developmental disturbances affect
morphogenetic processes both pre- and postnatally. Multi-
disciplinary studies of twins, with input from orthodontists,
molecular geneticists and twin researchers, hold great
promise for the future, not only in clarifying how genetic
factors contribute to oral diseases and disorders, but also in
unraveling the mysteries of how our facial symmetry is
determined.
CONCLUSION
With the advances that have occurred in both human
quantitative genetics and molecular biology over the past
decade, we are now in a position to build a more complete
understanding of how our genes and the environment
contribute to a range of dental diseases and malocclusions
that display multifactorial etiology.1 Far from being out-
dated, well-designed twin studies have a central role to play
in this new era. They can throw new light on how genes
influence developmental mechanisms.
Our challenge as dental researchers and orthodontists
will be to translate the knowledge we will soon have about
the genetic basis of disorders affecting the oral hard and
soft tissues into improved preventive and treatment strategies
for the community at large.
Observations by orthodontists of twin pairs presenting
at private or public dental clinics can also provide valuable
insights into how genes and the environment interact during
development. Finally, we can say that twin studies are here
to stay and they will keep throwing light on the relative
influence of genetics and environmental factors in the
etiology of malocclusion.
Twin Studies: Revealing the Genetic Basis of Malocclusion
Journal of Orofacial Research, January-March 2012;2(1):48-51 51
JOFR
REFERENCES
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3. The genetic contribution to orthodontic root resorption: A
retrospective twin study, Ngan, Daniel; Master Thesis. University
of Sydney.
4. Galton Francis; Natural Inheritance, London (1st ed) 1889.
5. Garn S Lewis, Polecheck D. Sibling similarities in dental
development. J Dent Res 1960;39:170-75.
6. Piacentini FD. An investigation of the genetic control of the
bones of the human craniofacial complex. Am J Orthod 1962;
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7. Kraus, Wise, Trai RM. Heredity and the craniofacial complex.
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roentgenographic cophalometry 1957.
11. Horowitz, Osborne, Degeorge. Hereditary factors in tooth
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12. Corruccini S Potter. Genetic analysis of occlusal variation in
twins. Am J Orthod 1980;8:140-45.
13. Goldberg S. Biometrics of identical twins from dental point of
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14. Shapiro BL. A twin study of palatal dimensions. Angle Orthod
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ABOUT THE AUTHORS
Dolly P Patel
Professor and Head, Department of Orthodontics and Dentofacial
Orthopedics, Government Dental College and Hospital, Ahmedabad
Gujarat, India
Bhaskar Gupta (Corresponding Author)
Senior Lecturer, Department of Orthodontics and Dento-
facial Orthopedics, Mahatma Gandhi Dental College and
Hospital, Jaipur, Rajasthan, India, Phone: +91-99291-66610
e-mail: bg_bhaskargupta@yahoo.co.in
Tanmay Sharma
Senior Lecturer, Department  of  Orthodontics and Dentofacial
Orthopedics, Mahatma Gandhi  Dental College and Hospital, Jaipur
Rajasthan, India


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