Inheritance and malocclusion / /certified fixed orthodontic courses by Indian dental academy

INHERITANCE

&
MALOCCLUSION
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INDIAN DENTAL ACADEMY
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 Basic

Principles in Genetics.
 Recent advances in genetics and molecular
biology.
 Inheritance and Malocclusion.
 Research objectives in Craniofacial
Genetics.
 Role of heredity in specific malocclusions.
 Genetics and Craniofacial Syndromes
 Orthodontic Implications.

Basic Principles and Terminology
in Genetics:
The science of genetics is concerned with the
inheritance of traits, whether normal or abnormal,
and with the interaction of genes and the
environment.
Consideration of the heritability of a particular
feature or trait requires a consideration of the
relationship between genotype and phenotype.

Genotype is defined as the genetic constitution of an
individual, and may refer to specified gene loci or to all
loci in general.
Phenotype of an individual is the final product of a
combination of genetic and environmental influences.
Phenotype may refer to a specified character or to all the
observable characteristics of the individual.
Heritability: The proportion of the phenotypic variance
attributable to the genotype.

Levels of Genetic Variation :
I. Specific traits
•Individual genotypes are readily identified and
differences are qualitative (discrete), for example,
the ABO blood antigen system.
• Gene frequencies can be estimated and the
Mendelian type of analysis can be applied.

II. Quantitative traits
•Traits such as height, weight, or tooth size
differences, characterized quantitatively between
individuals.
• More elusive to study because they are determined
by the alleles of many gene loci and, therefore, the
Mendelian type of analysis is not appropriate.
•Further modified by environmental conditions
which obscure the genetic picture.

Genetic differences caused by the segregation of many
genes is referred to as polygenic variation and the genes
concerned are referred to as polygenes.
They are subject to the same laws of transmission and
have the same general properties as the single genes
involved in qualitative traits, but segregation of genes is
translated into genetic variations seen in continuous traits
through polygenes.
If the genetic variation of a particular phenotypic trait is
dependent on the simultaneous segregation of many
genes and affected by environment it is referred to as
being subject to multifactorial inheritance.

Difficulties in determination of heritability for
multifactorial characters:
A feature of continuous variation is that different
individuals may occupy the same position on the continuous
scale for different reasons. e.g. Micrognathia can occur in:
Chromosomal disorders such as Turner's Syndrome
•Monogenic disorders such as Treacher Collins
or Sticklers Syndrome,

Syndrome

•An intra-uterine environmental problem, such as foetal
alcohol syndrome.

Concept of etiological heterogeneity:
The same gene defect can produce different phenotypic
anomalies, and syndromes can be due to defective gene
activity in different cells.
Conversely, different gene defects or combinations of
defective genes can produce a similar phenotypic
abnormality.
Genetic lethality or reduced reproductive fitness
can also complicate the diagnostic picture and
genomic imprinting can result in a gene defect
`skipping' a generation.

Categorization of Multifactorial Traits
I. Discontinuous multifactorial traits:
Traits determined by multiple gene loci, which are present or absent
depending on the number or nature of the genetic, and/or
environmental factors acting.
There is an underlying scale of continuous variation of liability to
develop the condition resulting from a combination of all the
genetic and environmental influences involved.
The condition is present only when the liability exceeds a critical
threshold value.

More than 20 discontinuous multifactorial traits have been described
in humans.
Cleft lip and palate is a congenital malformation inherited as a
discontinuous multifactorial trait.
In the mildest form the lip alone is unilaterally cleft, whereas in the most
severe form the lip is bilaterally cleft and the palatal cleft is complete.
The parents of a cleft lip and palate proband are often unaffected, and
there may be no family history of cleft lip and palate, but by producing
an affected child the parents are deemed to have some underactive genes
for cleft lip and palate formation.
Only when the balance exceeds a certain threshold will the malformation
occur, and the further the threshold is exceeded, the greater the extent of
the malformation.

II. Continuous multifactorial traits:
Many normal human characteristics are determined as continuous
multifactorial traits.
These traits by definition have a continuously graded distribution
e.g. for height there is a range from the very tall to the markedly
short with the mean of 169 ± 6.5 cm in English males (Connor and
Ferguson Smith, 1993).
The majority of individuals are centered around the mean. Such
distribution is characteristic of a continuous multifactorial trait.
Malocclusion should be regarded not as abnormal or as a disease,
but as a variation of occlusion in a continuous multi-factorial trait.

Recent advances in Genetics
and Molecular Biology:
Recent advances in molecular biology and in human
genetics have had a considerable influence in the
understanding of orofacial genetics.
Some insight into the genetic mechanisms involved in
craniofacial morphogenesis at the molecular level in the
embryo assists our appreciation of the role of genetics, not
only in the etiology of craniofacial abnormalities, but also
in the regulation of maxillary, mandibular, and tooth
morphology.

Facial development in the embryo is demarcated by the
appearance of the pre-chordal plate (the cranial end of the
embryo) on the 14th day of development.
One of the most unusual features of vertebrate facial
development is the origin of the facial mesenchyme, which
arises from neural crest cells.
Neural crest cells are a transient population of
embryonic cells that originate from the dorsal aspect of
the neural tube and migrate through the trunk and head in
vertebrates to form a diverse set of cell types.

The Cranial Neural Crest Cells arise from the
Rhombencephalon or hindbrain and migrate ventrolaterally
taking three pathways to produce craniofacial mesenchyme,
which get differenciated into cartilage, bone, cranial
neurons, glia and connective tissue.
Role of Homeobox genes:
Edward Lewis was the first person to identify the
homeotic genes in the fly, which help in controlling the
developmental response of groups of cells along the
body’s antero-posterior axis. In animals, these geneshomeobox genes, are regarded as master genes of the
head and face.

The first vertebrate homeobox gene was cloned in the
frog ( Xenopus levis) and was soon followed by cloning
in mouse.
The vertebrate genes are called HOX genes and consist
of 39 genes in humans as well as mice. They are
arranged in 4 different clusters- HOX A, HOX B, HOX
C, HOX D, on 4 different chromosomes.
As the neural crest cells migrate from the rhombomeres
to the specific branchial arches, they retain a specific
homeobox code, which specifies the form and pattern
of different derived regions of the head and neck.

The sub-families of the HOX genes which are of particular
interest in craniofacial patterning and morphogenesis, include:
Muscle segment (Msx)
Distal-less (Dlx)
Goosecoid (Gsc)
Otx gene.
Bar Class
Paired related genes (Prx and SHOT)
LIM homeobox gene.

The expressions of these genes are mediated through two main
groups of regulatory proteins-the Growth factor family and the
Steroid/Thyroid/ Retinoic acid Super family.
Some of the important regulatory molecules in the
mesenchyme, through which homeobox genes information is
expressed at the cellular level are:
Fibroblast Growth Factor (FGF)
Epidermal Growth Factor (EGF),
Transforming Growth Factors (TGFα, TGFβ)
Bone Morphogenetic Proteins (BMPs) .
The most complex part of CNC migration is the understanding
of how the combinations of HOX genes are expressed to
specify the fate of the cells.

Inheritance and Malocclusion
Familial tendencies are recognizable in the tilt of
the nose, the shape of the jaw and the look of the
smile.
Certain types of malocclusions run in families. e.g.
the Hapsburg jaw (the prognathic mandible of the
German and Austrian-Hungarian royal families), as
also the Class III malocclusion seen in some Eastern
Aleut families.

King Charles II of Spain who had a Hapsburg jaw.

Past studies of population, families and twins have
shown that genetic factors play an important role
in almost every aspect of craniofacial growth and
development, as also does environment.
The important question is not whether there are
inherited influences on the jaws and teeth,
because they obviously are, but whether
malocclusion is often caused by inherited
characteristics ?

Malocclusion could be produced by inherited
characteristics in two major ways:
Inherited disproportion between the size of
teeth and the size of the jaws: causing either
crowding or spacing.
Inherited disproportion between the size or
the shape of the upper and lower jaws:causing
improper occlusal relationships.

Dental anthropological evidence shows that
population groups that are genetically
homogeneous tend to have normal occlusion.
E.g.In pure racial stocks, such as the Melanesians
of the Philippine islands.
In heterogeneous populations, the incidence of jaw
discrepancies and occlusal disharmonies is
significantly greater. . E.g. Modern U.S. society.

Stockard (1941) carried out breeding
experiments with dogs and concluded that:
Individual features of the craniofacial complex
could be inherited according to Mendelian
principles independently of other portions
of the skull.
Jaw size and tooth size could be inherited
independently, and as genetically dominant
traits.

Chung, Niswander and Runck (1971) studied the
effect of intermixing of European, Japanese and
Chinese races with native Polynesians of Hawaii, on
the malocclusion in their descendents.
Their study showed that the effect of inter racial
crosses on malocclusion is additive rather than
multiplicative.

No independent inheritance of discrete morphologic
characteristics like tooth and jaw sizes.

Research Objectives in Craniofacial Genetics:
Smith and Bailitt ( Angle Orthod 1977) listed 5 main
research objectives in studying genetics and dental
occlusion.
Elucidating modes of inheritance.
Detecting the effects of admixture and inbreeding
Performing linkage analyses.


Estimating heritabilities.
Comparing population differences.

Linkage Analyses:
Linkage studies in humans have been mainly
restricted to studies of the sex chromosomes.
Cephalometric analysis of a sample of 47,XXY
(Klinefelter) males indicates pronounced facial
prognathism especially in the mandible.
Mandibular corpus length is significantly
increased and there is a tendency for reduction of
the cranial base angle.

 Studies

of 45, X females indicate a
retrognathic face, with short mandible and
flattened cranial base angle.



There is an increased prevalence of
crossbite, large maxillary overjet, distal
molar occlusion and a tendency to open
bite.

 The

X chromosome is thought to influence
growth of the cranial base at the
synchondroses, and also exerts a direct
effect on mandibular shape.

Estimation of Heritability:
 There

are two main analytic methods for
estimation of heritability of genetic traits

Twin studies and family line studies.
The twin method , gives geneticists one of the most
informative techniques available for analysis of
complex.genetic traits
The procedure is based on the fact that there are two
different types of twin pairs: Monozygous or
identical twins and dizygous or fraternal twins

Differences between members of a monozygous
pair are entirely non-genetic (environmental ) in
origin
Differences between dizygous twins, who share 50 %
of their total gene complement, are due to a
combination of genetic and environmental influences.
A prerequisite to the use of twins in studying the
genetic contribution to observed variation is a reliable
diagnosis of zygosity, and the accurate classification of
twin pairs as either monozygotic or dizygotic.

Methods used to classify twins.


Blood and serum group matching



Use of DNA markers, especially micro satellites.



Methods from classic anthropology i.e. on the basis of hair
color and texture, eye color, shape of chin, ears, nose and
mouth, dermatoglyphics , as well as skin color and texture,
as well as individual tooth morphology.



Comparison of anthropological and serological methods
show no divergence between the two approaches.

Data on Craniofacial
structure:
Hunter (1965) analysed cephalometric data from
35 dizygotic and 37 monozygotic twins.
Measurements of facial skeletal height
demonstrated a significantly higher component of
heritibility than did depth dimensions.
Similar findings have been reported by Lundstrom
and McWilliam (EJO 1987), Savoye et al (Angle
Orthod. 1998) and Manfredi et al (AJO1997)

 Kraus

et al (AJO DO 1959) found the shape and
growth of the individual bones of the skull and
face to be under strong genetic control, although
environmental factors have an important role in
determining the inter bone relationships.

 Nakata

et al (AJO 1973) also reported similar
findings.

 Naini

and Moss (AJO DO 2004) used 3D optical
surface scanning to assess genetic contribution to
external facial features in twins, and reported
significant genetic determination for the midface
region, as well as for vertical facial dimensions.

Data on Occlusal structure:


Lundström (1948) used the difference between quantified
occlusal variables in monozygotic (MZ) and dizygotic (DZ)
twins to measure genetic and nongenetic variance ratios.
Overjet was most highly heritable with a 3/1 ratio of genetic to
nongenetic variance (75 percent heritability).
Arch-width at Pl and Ml, buccal segment relation (sagittal
overjet), and overbite were less heritable
Generally, heredity and environment appeared about equally
important, but heredity was the major etiologic factor in the
severe malocclusions.



Corrucini et al ( AJO 1980) studied occlusal relationships in
60 pairs of twins



They reported that significant heritability of overjet, buccal
segment relationship, overbite, and total tooth
rotation/displacement could not be documented.



Arch size, individual tooth displacement scores, and crossbite, however showed significant genetic variance.



They concluded that environmental determination of occlusal
variation is roughly twice as important as was earlier thought.



Study by Corrucini and Sharma (EJO 1986) on Punjabi twins
revealed significant genetic control for dental arch and palate
dimensions, but environmental influences seemed important
for occlusal traits. www.indiandentalacademy.com

Data on Tooth structure and
dimensions:
 Potter

and Nance (1976) showed that tooth crown
dimensions were largely under genetic control.
Larger discordance in dizygotic twins than in
monozygotic twins provides strong evidence for the
existence of genetic control of individual
buccolingual and mesiodistal dimensions.

Studies on Punjabi twins showed substantial and complex
environmental determination for some dental dimensions,
especially in incisors and second molars.

Family line studies:


Korkhaus in 1931 showed a 4 generation family line which
exhibited mandibular protrusion.



Downs in 1928 reported on the heritability of Angle’s Class II type
of malocclusion.



Lebow and Swain in 1941 presented facial photographs of 7
generations of a family and concluded that repeated appearance of
long faces in successive generations cannot be from environmental
factors but from hereditary factors.



James Harris et al (AJO 1975) studied 77 families to investigate
and identify th mode of transmission of Class II division 1
malocclusion. Results showed that there is a polygenic mode of
inheritance.

 Nakasima

et al (1986) found that the
parents of those with a pseudo mesioocclusion had a similar prognathic skeletal
profile as do the parents of true mesioocclusion patients.

 This

indicates a familial tendency in the
development of pseudo as well as true
mesio-occlusion.


Population differences:


Relatively high frequency of Class II and low frequency of
Class III occlusion in North American Caucasian and
European populations.



Presence of the reverse situation in some groups of Asian
origin, including Polynesians, Alaskans, Aleuts, American
Indians and Pacific islanders.



Grewe (1968) showed that the tendency toward Classs II
relationships in North American Indians increased in
relation to the proportion of Caucasian ancestry. A similar
effect was seen in Polynesian-Caucasian hybrids.

Higher prevalence of bimaxillary protrusion of the jaws
in African people as compared to Caucasians.

Role of Heredity in Specific
Malocclusions:
 Etiology

of Crowding and Malalignment:

Continuing evolutionary reduction in jaw size.
Additive effects on malocclusion due to out
breeding.
Other important factors may be environmental e.g.
alterations in diet, mouth breathing.


 Class

II Division 1 Malocclusion:



The mandible is significantly more retruded than in Class I
patients, with the body of the mandible smaller and overall
mandibular length reduced.



Higher correlation between the patient and his immediate
family than among random pairings of unrelated siblings.



This supports the concept of polygenic inheritance for
Class II division 1 malocclusion.



Environmental factors can also contribute to the etiology
e.g. lip/tongue contact for an anterior oral seal, digit
sucking habits, lip incompetence

Class II Division 2 Malocclusion:


Distinct clinical entity which may be considered as a
syndrome



Unique combination of deep overbite, retroclined incisors,
Class II skeletal discrepancy, high lip line with strap-like
activity of the lower lip, and active mentalis muscle.



Often accompanied by a poorly developed cingulum on the
upper incisors and a characteristic crown root angulation.
Peck et al. (1998) also describe characteristic smaller than
average teeth when measured mesio-distally.





Tendency to a forwardly rotating mandibular development
which in turn, has an influence on the position of the lower
lip relative to the upper incisors, and an increase in
masticatory muscle forces



Familial occurrence of Class II division 2 has been
documented in several published reports including twin
and triplet studies e.g. Korkhaus (1930), Markovic (1992),
Peck et al. (1998).



Markovic (1992) carried out a clinical and cephalometric
study of 114 Class II division 2 malocclusions (48 twin
pairs and six sets of triplets)



Of the monozygotic twin pairs, 100 per cent demonstrated
concordance for the Class II division 2 malocclusion, while
almost 90 per cent of the dizygotic twin pairs were
discordant.



This is strong evidence for genetics as the main etiological
factor



Genetic influence is probably autosomal dominant with
incomplete penetrance and variable expressivity.



It could also be explained by a polygenic model with a
simultaneous expression of a number of genetically
determined morphological traits acting additively.



Controversy regarding the etiology arises from a failure to
appreciate the synergistic effects of genetics and
environment.



Ballard (1963), Houston (1975) Mills (1982), and others
considered that a high lip line, and a particular lip
morphology and behavior were the main etiologic factors.



(Lauweryns et al. (1995) found strong genetic factors in
certain aspects of masticatory muscle behavior.

Class III Malocclusion:


Strohmayer (1937) concluded from pedigree analysis of the
Hapsburg family line that the mandibular prognathism was
transmitted as an autosomal dominant trait.



Suzuki (1961) studied 243 Japanese families and noted that
there was a significantly higher incidence of the trait in
family members of cases who had mandibular
prognathism.



Schulze and Weise (1965) reported that concordance for
mandibular prognathism in monozygotic twins was six
times higher than among dizygotic twins.



Both of the above studies report a polygenic hypothesis as
the primary cause for mandibular prognathism.



A Class III malocclusion may result from deficiency in
maxillary growth, excessive mandibular growth, or a
combination of both.
Influence of a distinctive cranial base morphology with a
more acute cranial base angle and shortened posterior
cranial base resulting in a more anterior position of the
glenoid fossa, thus contributing to the mandibular
prognathism (Ellis and McNamara, 1984; Singh et al.,
1997)
Environmental factors e.g.enlarged tonsils, nasal blockage,
congenital anatomic defects, hormonal disturbances,
endocrine imbalances, posture, trauma/disease, including
premature loss of the first permanent molars.

 Litton

et al. (1970) proposed a polygenic
model with a threshold for expression to
explain familial distribution, and the
prevalence within the general population
and in siblings of affected persons.

 They

also made the suggestion that different
modes of transmission might be operating in
different families or different populations.

 Soft

tissues do not generally play a part in
the etiology of Class III malocclusion, and
may in fact have a compensatory effect.



Hypodontia and microdontia.
 Studies

of different populations indicate a wide
variability in the incidence of hypodontia.

 Some

authors have reported the permanent
maxillary lateral incisor as most commonly
missing, while others have reported the
mandibular 2nd premolar as being so.

 Agenesis

of teeth is generally related to an overall
reduction in tooth size. Hence, hypodontia and
microdontia tend to occur in the same children.

 Hypodontia

is, to a great degree, genetically
determined, and transmitted by an autosomal
dominant inheritance with incomplete penentrance
and variable expression.

 Environmental

factors may also play a role.

A

mutation in the homeobox gene MSX1 has been
implicated as a factor in familial selective agenesis
of second premolars and third molars.

 The

increased incidence of hypodontia in children
with clefts suggests that the same etiologic factors
might be responsible for both the anomalies in
affected children.www.indiandentalacademy.com

Maxillary Canine Impaction


Buccal impaction of the maxillary canine is usually due to
inadequate arch space and it eventually results in eruption
in most cases.



In contrast, palatal displacement of the maxillary canine is
a positional anomaly that generally occurs despite
adequate arch space.



Peck, Peck and Kataja (Angle Orthod 1994) support a
genetic etiology for the palatally displaced canine (PDC)
on the basis of five evidential categories:



1.

Occurrence of other dental anomalies concomitant with
PDC

2.

Bilateral occurrence of PDC

3.

Sex differences in PDC occurrence.

4.

Familial occurrence of PDC.

5.

Population differences in PDC occurrence.
From analysis of available evidence, the PDC positional
anomaly appears to be a product of polygenic,
multifactorial inheritance.

Tooth Transpositions.
Def: Positional interchange of two adjacent teeth –
particularly of the roots – or the development or eruption
of a tooth in a position occupied normally by a nonadjacent tooth.
The Maxillary Canine-1st premolar transposition has been
the most frequently reported type of transposition, with a
low prevalence rate ranging from 0.03% (Sweden) to
0.25% (Scotland).
Peck, Peck and Attia (Angle Orthod 1993) gathered data
from 43 cases with this anomaly. They found a number
of factors to be associated with it.

Increased frequency of associated dental anomalies
(tooth agenesis, 37%, and peg-shaped maxillary
lateral incisors, 16%).
Bilateral occurrence in a high percentage of cases
(23%).
Familial occurrence (one pair of identical twins).
Different pattern of occurrence in males and
females (females predominate, 34 to 9).
Different frequency of occurrence likely in
different populations (40 whites and 3 blacks).
It appears that the Mx.C.P1 transposition anomaly
is polygenic in origin, fitting into the broader
pattern of multifactorial inheritance.

Midline diastema.








Maxillary midline diastema (MMD) is a relatively
common dental malocclusion characterized by a space
between the maxillary central incisors, with functional and
esthetic consequences.
The literature strongly supports racial differences in the
distribution of the trait, with blacks demonstrating
consistently higher prevalence values than Whites, Asians,
or Hispanics.
Nainar and Gnanasunderam( Angle Orthod 1999) mention
that familial incidence was 1 of 3 significant factors
associated with the prevalence of MMD.
Shashua and Artun ( Angle Orthod 1999) reported that a
family history of diastema was 1 of 2 significant factors
for diastema relapse after orthodontic correction.

 Gass,

Valiathan, Tiwari, Hans and Elston
(AJODO 2003) reported that MMD is more
heritable in Whites than in Blacks.

 Environmental

influences such as periodontal
drift, unreported habits, inaccurately reported
missing teeth, or excessive incisor proclination
might be responsible for higher prevalence rate in
Blacks.

 The

data from their study also suggested an
autosomal dominant mode of inheritance of
maxillary midline diastema.

Genetics and Craniofacial
Syndromes.
Holoprosencephaly:
Malformation sequence in which impaired
midline cleavage of the embryonic forebrain is
the main feature.
Associated facial anomalies in this syndrome
include cyclopia, arhinia, median cleft lip,
lateral cleft lip.

A number of mutations in several genes have
been identified as causative, of which the best
known is Sonic Hedgehog (SHH).
Other gene mutations involve TGIF, SIX3,
PTCH.

 Craniosynostosis:
 Defined

as the premature fusion of skull bones. It
is found in association with a number of different
disorders.e.g. Crouzon, Apert, Jackson-Weiss,
Pfeiffer, Beare-Stevenson, and Saethre-Chortzen
syndromes.

 Crouzon

Syndrome: Characterized by
craniosynostosis of coronal sutures leading to
brachycephaly.Additionally, there is a high
prominent forehead, hypertelorism, strabismus,
midface hypoplasia, prominent beaked nose, high
arched palate, mandibular prognathism, dental
malocclusion. www.indiandentalacademy.com

Typical picture of
Crouzon Syndrome.

 Genetic

etiology: Mutations in the Fibroblast
Growth Factor Receptor (FGFR) genes with
resultant gain of function, especially in FGFR2
have been identified in various craniosynostosis
syndromes.

 Also

chromosomal alterations leading to loss of
function in the TWIST gene, are implicated.


Treacher Collins Syndrome
 Autosomal

dominant
disorder consisting of
bilateral zygomatic
hypoplasia, downslanting
palpebral fissures,
dysplastic ears,
downturned mouth and
hypoplastic mandible.


 Cleft


palate may also be present.

Malocclusion is common, and hypoplastic
teeth as well as open bite may be present.

 Genetic

Etiology: Mutations in the TCOF1

gene.


Orofacial Clefting.
Much evidence supports the role of genetics in
orofacial clefting.
Concordance for MZ twins with cleft lip-palate(40%)
is far greater than for DZ twins (4.2%).
Various factors have been implicated such as major
genes, minor genes, environmental factors, and
developmental threshold.
Two important genes implicated in clefting are
MSX1 and TGFß3

Orthodontic Implications:








Each malocclusion occupies its own distinctive slot in the
genetic/environmental spectrum.
The goal of diagnosis includes determination of the
relative contribution of genetics and environment.
The greater the genetic component, the worse the
prognosis for a successful outcome by means of
orthodontic intervention.
There is also, currently a lack of evidence to show that
orthopedic appliances can influence the growth of skeletal
bases significantly beyond their innate genetic potential.
Further advances in the field of molecular genetics should
contribute to a better understanding of heritability of
malocclusions in the future.

References









Mossey P.A. The Heritability of Malocclusion: Part 1.
BJO 1999;26(2): 203-213.
Mossey P.A. The Heritability of Malocclusion: Part 2.
BJO 1999; 26(3):195-203.
Smith R.J., Bailit H. Problems and methods in Research on
the genetics of Dental occlusion. Angle Orthod. 1977;
47(1): 65-81.
Townsend G, Aldred M, Bartold P. Genetic aspects of
dental disorders. Austr. Dent J 1998;43(4):269-286.
Krishnan V. Neural Crest cells, Homeobox genes and
craniofacial development. J Ind Orthod Soc 2002;35: 4250.
Markovic M. At the crossroads of oral facial genetics. EJO
1992; 14: 469-481.










Nainar SMH, Gnanasundaram M. Incidence and etiology
of midline diastema in a population in South India. Angle
Orthod 1989;59:277-82.
Gass J, Valiathan M, Tiwari H, Hans M, Elston
R.Familial correlations and heritability of maxillary
midline diastema AJODO 2003 123 (1):35-39.
Corrucini RS, Potter R.Genetic analysis of occlusal
variations in twins. AJO 1980;78: 140-154.
Lauweryns I. The use of twins in dentofacial genetic
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