GOOD MORNING

HEREDITY AS ETIOLOGY OF

MALOCCLUSION

BY-

DR. PARAG S. DESHMUKH

IST

MDS

INTRODUCTION :

• A sound knowledge of various etiologies or causes of malocclusion is a basic requirement for any orthodontic student.

• Malocclusion is an interaction between genetic and environmental factors.

• Malocclusion may result from complex interaction among multiple factors that influence growth and development.

• Broadly speaking, malocclusions are either caused by genetic or environmental factor

• It is of great value in making preventive and interceptive orthodontic procedures as possible because malocclusion is prevented or intercepted by timely removal of cause.

WHITE AND GARDINER'S CLASSIFICATION :

A. Dental base abnormalities

1. Antero-posterior mal relationship 2. Vertical mal relationship 3. Lateral mal relationship 4. Disproportion of size between teeth and basal bone 5. Congenital abnormalities.

A. Pre eruption abnormalities 1.Abnormalities in position of developing tooth germ 2. Missing teeth 3. Supernumerary teeth and teeth abnormal in form . 4. Prolonged retention of primary teeth 5. large labial frenum 6. Traumatic injury

C. Post-eruption abnormalities1. Muscular

a. Active muscle forceb. Rest position of musculaturec. Sucking habitsd. Abnormal path of closure

2. Premature loss of deciduous teeth

3. Extraction of permanent teeth

MOYER’S CLASSIFICATION :

1.Heredity – a. Neuromuscular b. Bonec. Teethd. Soft Parts

2. Developmental defects of unknown origin3. Trauma

a. Prenatal trauma and birth injuriesb. Postnatal trauma

4. Physical agents a. Premature extraction of primary teeth b. Nature of food

5. Habits:

a. Thumb sucking and finger sucking

b. tongue thrusting

c. lip sucking and lip biting

d. posture

e. nail biting

f. abnormal swallowing and other habits

6. Diseases a. systemic diseasesb. endocrine disordersc. local diseases

i. Nasopharyngeal diseases and disturbed respiratory function ii. Gingival and periodontal diseases iii. Tumors

iv. Caries7. Malnutrition

GRABER’S CLASSIFICATION:

GENERAL FACTORS1. Heredity2. Congenital3. Environment

a. Prenatalb. Post natal (birth injury, cerebral palsy, T.M.J. injury)

4.Predisposing Metabolic disease a. Endocrine imbalance b .Metabolic disturbances c. Infectious diseases5. Dietary problems (nutritional deficiency)

6. Abnormal pressure habits and functional aberrations a. Abnormal sucking b. Thumb and finger sucking c. Tongue thrust and tongue sucking d. Lip and Nail biting e. Abnormal swallowing habits f. Speech defects g. Respiratory abnormalities h. Tonsils and adenoids i. Psychogenic tics and bruxism7.Posture8.Trauma and accidents

LOCAL FACTORS:

1. Anomalies of number: Supernumerary teeth, Missing teeth [congenital absence or loss due to accidents. caries. etc.]2. Anomalies of tooth size3. Anomalies of tooth shape4. Abnormal labial frenum5. Premature loss of deciduous teeth

6. Prolonged retention of deciduous teeth7. Delayed eruption of permanent teeth8. Abnormal eruptive path9. Ankylosis10. Dental caries11. Improper dental restoration

Houston’s classification:

General Factors: Discrepancy between tooth size & arch size resulting in crowding or spacing, skeletal mal relationships and soft tissue factors. Local Factors:1. Anomalies in number of teeth Extra teeth Missing teeth Loss of permanent teeth Premature loss of deciduous teeth Retained deciduous teeth2. Anomalies in form & position of teeth3. Habits4.Abnormal labial frenum.

Proffit’s classification:

Specific causes of malocclusionA) Disturbance in embryologic developmentB) Skeletal growth disturbance (i) fetal molding and birth injuries eg: intrauterine molding :birth trauma to mandible (ii)childhood fractures of jawsC)muscle dysfunctionD)acromegaly and hemi mandibular hypertrophyE)disturbances of dental development

F. Genetic influencesG. Environmental influencesa) equilibrium theory and development of dental occlusionb) functional influences on dentofacial development

GRABER’S CLASSIFICATION:

A. General factors:

1.Heredity2.Congenital defects3.Environment a. prenatal b. postnatal4. Predisposing metabolic climate and disease. a. endocrine imbalance b. metabolic disorders

5. Nutritional deficiency

6. abnormal pressure habits and functional abrasion a. tongue thrust and tongue sucking b. thumb and finger sucking c. lip and nail biting

Local factors:

1. Abnormalities of number: supernumerary teeth missing teeth 2.Anomalies of tooth shape

3.Abnormal labial frenum

4.Premature loss

5.Prolonged retention

6.Delayed eruption of permanent teeth.

7.Abnormal eruptive path

8.Ankylosis

9.Dental caries

10.Improper dental restorations.

Basic terms: Genetics -

Branch of biology that deals with heredity and variations.

Heredity –transmission of character from one generation to next or from parents to offspring.

Variation –forces or influences due to which no two organisms are exactly alike.

Trait –An inherited character and its detectable variant.

Inheritance – Transmission of characters from parents to offspring.

Alleles or Allomorph:

• Genes like chromosomes are arranged in pair.

• Each pair of gene control a trait or character.

• These pair of gene is called allele.

Chromosomes:

Chromatin of the interphase nucleus is closely coiled in the form of rod like basophilic structures called chromosomes.

DNA: (Deoxyribo-nucleic acid)

Chromosomes are made up of two long chains of DNA, intertwined and twisted in a specific sequence.

GENES:

• Genes – (from Greek word ‘Gene’- to become)• It is particular segment of DNA which is responsible

for the inheritance and the expression of the character.

• These are the structural units of heredity stored in chromosomes.

• They contain the heredity information encoded in their chemical structure for transmission from generation to generation.

• They have ability to determine traits. E. g. tooth size, shape, arch form etc.

Mutation:

• This term is used to denote a physico-chemical change in a gene which alters the effect on character or trait.

Homozygous:When two members of pair of genes are alike.

Heterozygous:When two members of pair of genes are different

Dominant:

• It is in allele that represent its trait in presence of alternative allele i.e. in heterozygous condition

Recessive:

• It is an allele which is not expressed in presence of alternative allele.

HEREDITY:• Heredity means transfer of characters or traits from parents to

offspring.

• Heredity plays part in the following conditions:

I. Facial asymmetriesII. Micro and macrognathiaIII. Micro and macrodontiaIV. Anodontia, Oligodontia, hypodontia.V. Deep biteVI. Crowding VII. Rotations etc.

Types of transmission of malocclusion:

Malocclusions are transmitted by three ways:

i) Repetitive: Recurrence of single dentofacial deviation within immediate family.

ii) Discontinuous : recurrence of tendency for malocclusal trait to reappear after few generations.

iii) Variable: Expression of different but related types of malocclusion within the several generations of the same family.

Modes of Inheritance:

Following are the modes of inheritance:

•Autosomal dominant

• Autosomal recessive

• Sex-linked

• Polygenic

•Chromosomal

Genetic studies: Twin study:

• In this study twins are compared.

• Comparing monozygotic twins with dizygotic twins is the best way to determine the extent of genetic effect On malocclusion.

Monozygotic twins• Happen due to early division of fertilized egg.• Both the individuals will have the same DNA• They are genetically similar.• Also called identical twinsDizygotic twins• Happen when two eggs fuse with two different spermatozoa• The twins will have different DNA• They are genetically dissimilar• Also called fraternal twins.

Procedure:

• The heritability of the malocclusion can be determined by comparing the monozygotic twins, dizygotic twins and ordinary siblings.

• In monozygotic twins, any change in occlusion or feature could be attributed to environment factor since both have same DNA.

• In dizygotic twins, interplay of genetic and environmental factor is studied.

Disadvantages of twin studies:

• Difficulty in identifying identical twins.• Difficulty to establish the same environment for the twins.

Family study/pedigree study:

• In this method of study occlusal features and differences between mother- child, father-child and siblings are analyzed.

• Helps to differentiate between dominant and recessive traits

• Dominant traits will be expressed in all the subsequent generation.

Butlers field theory

The human dentition is divided into four fields:

(i) incisor (ii) canine (iii) premolar(iv) molar.

• The most distal tooth in each field is the most susceptible to changes or variations.

• The changes include –

Absence of tooth Variation in size Shape and structure.

• Accordingly lateral incisors, second premolars and third molars are the most variable in their group.

• Canine is the least variable tooth in the arch.

• Butlers field theory does not apply in lower anterior region, where mandibular central incisor is more commonly missing than lateral incisor.

Advances in genetics:

• Gene mapping: -Mapping of genes to specific locations on chromosomes.

• Cloning: -Clone is a series of identical DNA.

• Gene therapy: -Insertion of normal genes in the bodies of individuals affected with genetic disorder.

• Lauweryns et al(1993) summarized a number of twin studies and concluded that 40% of the dental and skeletal variations that lead to malocclusion can be attributed to hereditary factors.

The use of twins in dentofacial genetic research Lauweryn I,Carels C,Vlietinck R AJODO 1993;103:33-38

• According to Bolton –Brush growth study (1930s to 1970’s) Harris and Johnson concluded that the heritability of skeletal characteristics was relatively high but that of dental (occlusal) characteristics was low.

There is a strong influence of inheritance on facial featuresEg: tilt of the nose, the smile, shape of jaw.

Certain type of malocclusion run in families.

Mcuigan described the most well known example of inheritance, the Hapsburg family having the distinct characteristic of prognathic lower jaw known as Hapsburg jaw of German royal family.

Litton et al in 1970 concluded that the dental Class III characteristic were related to genetic inheritance in offspring and siblings.

• We can say that there is definite genetic determinant that influences the ultimate accomplishment of dentofacial morphology.

• The pattern of accomplishment has a strong hereditary component.

• Since the offspring is a product of dissimilar heredity, cognizance must be taken of the inheritance from both the sources.

• A child may have resemblance to father or mother or it can be combination of features from each parent.

• i.e. child may inherit tooth size and shape, jaw size, shape and relationship from the father or mother.

• But it is equally possible that child may inherit tooth shape and size characteristic from one parent and jaw size and shape from other.

• In the complex interplay of chromosomes and genes, two recessive factors may combine to become a dominant characteristic, or dominant characteristic may be offset by genetic potential from other parent; and characteristic may then disappear in the offspring.

HEREDITARY RACIAL INFLUENCE:• DENTAL CHARACTERISTICS, LIKE FACIAL CHARACTERISTICS SHOW RACIAL

INFLUENCE.

• WHERE THERE HAS BEEN MIXTURE OF RACIAL STRAINS THE INCIDENCE OF

JAW SIZE DISCREPANCIES AND OCCLUSAL DISHARMONIES IS SIGNIFICANTLY

GREATER.

• Professor Stockard did his experiments on dogs to show the role of genetics in development of malocclusion.

Stockard’s study on cross-bred dogs (1930s)• Observations: Dramatic malocclusions did occur more from jaw discrepancies than

from tooth-size- jaw-size discrepancy.

This seemed to confirm that independent inheritance of facial

characteristics could be a major cause of malocclusion.

Study was misleading as many breeds of small dogs carry genes for

Achondroplasia.

HEREDITARY FACIAL TYPE:• THE FACIAL TYPE OF THE OFFSPRING IS PROBABLY, HEAVILY

INFLUENCED BY HEREDITY.

• HASUND AND SIVERTSEN POINT OUT THE SEX LINKED NATURE OF

FACIAL WIDTH AND DENTAL ARCH SHAPE.

• FEMALES DEMONSTRATE A POSITIVE CORRELATION – THE WIDER

THE FACE THE WIDER THE ARCH.

HEREDITARY INFLUENCE ON GROWTH AND DEVELOPMENTAL PATTERN:• AS THE MORPHOGENETIC PATTERN HAS A STRONG HEREDITARY

COMPONENT, IT IS REASONABLE TO ASSUME THAT THE

ACCOMPLISHMENT OF THAT PATTERN IS ALSO AT LEAST

PARTIALLY UNDER THE INFLUENCE OF HEREDITY.

• OBVIOUSLY THE ENVIRONMENTAL INFLUENCES ARE IMPORTANT

HERE, TOO, AND THEY CAN AND MODIFY HEREDITARILY

DETERMINED PATTERN.

Heredity and specific dento-facial morphologic characteristic:

• Lundstorm made an intensive analysis of some characteristics in twins, and he concluded that heredity could be considered significant in determining the following characteristics:

Tooth sizeWidth and length of arch Height of the palateCrowding and spacing of the teeth Degree of sagittal overbite. (overjet)

If heredity influence is present and can be demonstrated in the enumerated areas, it is logical to assume that heredity plays a part in the following conditions.

Congenital deformities Facial asymmetries Macrognathia and micrognathia Macrodontia and microdontia Oligodontia and anodontia Cleft palate and harelip Deep overbite Crowding and rotation of teeth Mandibular retrusion Mandibular prognathism

Hereditary causes can be those

influencing the :• Neuromuscular system

• Dentition

• Skeletal structure

• Soft tissues other than musculature.

NEUROMUSCULAR SYSTEM:

• The anomalies that have been found to possess some inherited component include deformities in size, position, tonicity, contractility, and in the neuromuscular coordination pattern of facial, oral and tongue musculature.

• Certain tongue & lip habits can be learned by imitation or be inherited and may be found to reoccur within a family over generations.

DENTITION

• Certain characteristics, especially related to the dentition are definitely inherited. These include:

A. Size and Shape of the Teeth:

Studies on twins have proved that the size and relative shape of the teeth is inherited e.g. Peg shaped.

• Lateral are the most commonly seen and noticed abnormally shaped teeth encountered clinically.

• To produce a malocclusion the discrepancy should exist between the basal bone and the teeth size, or the associated skeletal structures.

B. Number of teeth:

• Presence of either more or less no. of teeth can also be inherited.

• This includes condition such as Anodontia, Oligodontia, and Hypodontia.

C. Primary Position of Tooth, Germ and the Path of Eruption is considered to be inherited. E.g. Cross bites, Ectopic tooth eruption.

.

D. Shedding of Deciduous Teeth & Sequence of Eruption

E. Mineralization of Teeth:

Inherited defects of the tooth structure differ from defects in mineralization as they are present in both the deciduous dentition as well as permanent dentition and are localized in the enamel or the dentine. e.g. hypo plastic teeth

SKELETAL STRUCTURES:

Skeletal structures are partially inherited e.g. Class III skeletal pattern is associated with familial tendency

SKELETAL CLASS III

SOFT TISSUES (OTHER THAN THE NEUROMUSCULATURE):

• These generally includes the size & shape of the frenum especially the maxillary labial frenum.

GENES IN MALOCCLUSION

Some of the key genes required for craniofacial morphogenesis

• Polarizing signals: Shh, Bmp2, Bmp4 and Bmp7, Wnt5a, Smad2–4

• Growth factors and receptors : Egf, Egfr, Tgfa, Tgfb1–3, Fgf1, Fgf2, Fgf8, Fgfr1, Fgfr2

• Transcription factors: Hoxa2, Irf6, Lhx8, Pax9, Pitx2, Prx1, Msx1, Tbx1, Tbx22

• Cell adhesion molecules: Pvrl1, Connexin43, E-cadherin

• Extracellular matrix: Mmp2, Mmp3, Mmp9,Mmp13, Timp1–3

In humans number of homeobox containing genes are expessed in maxillary, mandibular arches and the developing facial primordia.

• THESE GENES INCLUDE:

MSX1 AND 2 DLX1-6 BARX-1 PAX SHH

Homeobox genes:

• Homeobox genes are genes, which are highly conserved through out evolution of diverse organisms and are now known to play a role in patterning the embryonic development.

• These can be regarded as master genes of the head and face controlling patterning, induction, programmed cell death & epithelial mesenchymal interaction during development.

Those of particular interest in cranio facial development include:

• Hox group

• Msx 1 and Msx 2 (muscle segment)

• Dlx ( distalless )

• Otx ( orthodontical )

• Shh ( sonic hedgehog )

MSX genes: (Muscle segment homeobox gene)

• Involved in tooth formation.

• This gene has been associated with dental and facial abnormalities.

• MSX-1 genes are expressed in migrating neural crest cells and later in the mesenchymal cells of dental papilla and follicle.

• MSX-2 genes are involved in signaling interactions, which are essential for the tooth development.

DLX genes(Distal-less homeobox gene)

• Expressed in migrating neural crest cells and the first branchial arch.

• Expression of DLX-1 and DLX-2 in maxillary and mandibular arch mesenchyme is restricted to the proximal regions where the future molar teeth will develop.

BARX genes: (Bar class homeobox genes)

• BARX genes along with DLX-2.

• No expression in anterior region.

• As tooth development proceeds, BARX-1 expression becomes localized extensively to the mesenchymal region around developing molars.

• Mutation of these genes could be associated with facial and dental anomalies.

PAX genes: (paired-box homeotic genes)

• There are nine PAX genes arranged in 4 groups.(PAX-1 to PAX-9).

• PAX-9 is associated with development of teeth.

• Mutation in this gene results in conditions such as Hypodontia, transposition, etc.

• Nubuser et al. found that PAX 9 transmission factor is associated with the genetic mechanism for tooth displacement anomalies, such as palatally displaced canines and canine transposition.

SHH genes: (Sonic hedgehog genes)

• Expressed in the epithelial thickenings of tooth forming regions.

• SHH along with BMP-4 determine the position of future forming tooth germs.

• SHH is necessary for initiation of tooth development, epithelial signaling and cuspal morphogenesis.

• GLI Zinc transmission factors are known to act downstream of SHH gene.

• GLI1-3 play a vital role in development of tooth mutation of GLI results in formation of abnormal tooth.

PTHR gene:

• These genes are included in eruption of teeth.

• Mutation in PTHR 1 causes primary failure of eruption.

• Mutation in PTHR is strongly associated with failure of orthodontically assisted eruption and tooth movement.

Primary failure of eruption and PTHR1: the importance of genetic diagnosis for orthodontic planning Sylvia A. Frazier-Bowers, Darrin Simmons, J. Timothy Wright, William R. Proffit, James L. Ackerman(American Journal of Orthodontics and Dentofacial Orthopedics, Volume 137, Issue 2, February 2010, Pages 160-161)

TBX22:

These genes play essential roles in early development and in particular mesoderm specification

Human Molecular Genetics, 2004, Vol. 13, Review Issue 1

PVRL1:

• mutations were identified in the cell adhesion molecule PVRL1 (Nectin-1), which is expressed in the developing face and palate Autosomal recessive CLP with ectodermal dysplasia (CLPED1)

IRF6:

• In the mouse, Irf6 expression is restricted to the palatal MEE immediately prior to and during fusion

MSX1:

• MSX1 first came to prominence as a candidate for CL/P following the generation of a gene knockout with cleft palate and Oligodontia.

• Jezewski et al analysed a large cohort of CL/P patients from a variety of different ethnic origins and demonstrated that up to 2% of patients, predominantly with CLP, carried MSX1 mutations

(Human Molecular Genetics, 2004, Vol. 13, Review Issue 1)

(Kanzaki and colleagues)

• Gene therapy with OPG and RANKL Local RANKL gene transfer to the periodontal tissue accelerated orthodontic tooth movement by approximately 150% after 21 days, without eliciting any systemic effects

• With OPG gene transfer, there is inhibition of the tooth movement by 50% after 21 days

(Orthodontics in the year 2047: genetically driven treatment plans, Bruce Havens, Sunil Wadhwa, Ravindra Nanda, 549 – 56 vol XLI Number 9, JCO/ Sep 2007)

Use of functional appliances ↓ses regulation of the genes ( PTHrP, Indian Hedgehog, Collagen typeX and VEGF) in the mandibular condylar cartilage

(Orthodontics in the year 2047: genetically driven treatment plans, Bruce Havens, Sunil Wadhwa, Ravindra Nanda, 549

56 vol XLI Number 9, JCO/ Sep 2007)

• The size of the mandible (as well as that of the maxilla) is partially regulated by the number of neural crest cells that migrate successfully into the first pharyngeal arch.

• Mutations in genes such as TREACLE may be responsible for the milder cases of mandibular retrognathia commonly seen in orthodontic practice

(Orthodontics in the year 2047: genetically driven treatment plans, Bruce Havens, Sunil Wadhwa, Ravindra Nanda, 549 – 56 vol XLI Number 9, JCO/ Sep 2007)

Contribution of genetics in various malocclusions:

Class II div 1 Malocclusion:

• Various investigations show that mandible is significantly retruded & the overall mandibular length is reduced in most of the class II div 1 malocclusions.

• A higher correlation between the patient and the family members is found suggesting an obvious genetic etiology.

 

• Some environmental factors have also been suggested to play a role in establishing the class II div 1 pattern. A few are listed below;

1. Soft tissue – controls the position of upper & lower incisors producing a class II div 1 pattern of incisors.

2. Digit sucking.

3. Lip incompetence.

Bolton & Brush Growth study(conducted on siblings between 1930s & 1970s).

Harris & Johnson concluded that-heritability of craniofacial characters was high but dental was low.

Heritability estimates for skeletal characters increases with age but for dental characters decreased-indicates environmental contribution.

Heritability of craniometric & occlusal variables: A longitudinal sib analysis Edward F Harris & Michelle G.Johnson AJO-DO 1991;99:258-68.

• Harris has shown that the craniofacial skeletal pattern with class II malocclusion is heritable & there is high resemblance of skeletal pattern in their siblings.

• He concluded that the genetic basis for their resemblance is polygenic.

Inheritance of Class II malocclusion in three brother

Class II div 2

• Markovic in 1992 after evaluating 48 twin pairs came to the

conclusion that the concordance rate for this malocclusion in MZ

twins was 100%.

• Whereas in DZ twins the concordance rate was only 10% and 90%

were discordant.

• This is a strong evidence to quote genetics as a main etiologic factor.

• Familial occurrence of Class II division 2 has been documented in several published reports including twin and triplet studies. (E.g. Kloeppel, 1953: Markovic, 1992) and in family pedigrees from Korkhaus (1930), Rubbrecht (1930), Trauner (1968) and 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.

• Intra- and inter- pair comparisons were made to determine concordance/ discordancy rates for monozygotic and dizygotic twins

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

• This is strong evidence for genetics as the main aetilogical factor in development of class II div 2 malocclusion.

Class III

• One of the famous examples was the mandibular prognathism that was running in

the Austrian monarchy - The Hapsburg jaw.

HAPSBURSG JAW

CHARLES IITHE BEWITCHED

PHILIP IV HAMB

• Strohmayer in 1937 concluded from detailed pedigree analysis that

the mandibular prognathism was transmitted as an autosomal

dominant trait.

Suzuki (1961) studied 1362 persons from 243 Japanese families and noted that-

• While the index cases had mandibular prognathism, there was a significantly higher incidence of this trait in other members of his family (34.3 per cent) in comparison to families of individuals with normal occlusion (7.5 per cent).

• Schulze & Weize in their twin studies in 1965 concluded that the concordance rate in MZ twins was 6 times higher than that in DZ twins.

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

• A wide range of environmental factors have been

suggested to play a role in class III malocclusions. To

name a few–

1. Enlarged tonsils

2. Nasal blockage

3. Congenital anatomic defects

4. Hormonal disturbances

5. Posture etc.

Genetic influence on tooth no & size

• Osborne et al (1958) in their twin studies concluded that crown dimensions are

strongly determined by heredity.

• Clinical evidence suggests that congenital absence of teeth and reduction in tooth

size are associated

e.g. hypodontia and hypoplasia of -

maxillary lateral incisors frequently present simultaneously.

• This implies that they are different expressions of the same disorder.

• It is apparent from the study that tooth size fits the polygenic multi factorial

threshold model.

Supernumerary tooth:

• Most frequently seen is a mesiodens in the premaxillary region with a male predilection.

• They are most commonly seen in parents & siblings of patients.

• Sugaku (1963) analysed the data from family studies and suggested that the genetics of the less prevalent condition of supernumerary teeth is under the control of a number of different loci.

Hypodontia:

• Study of children with missing teeth found that up to half of their siblings & parents

also had missing teeth.

• Markovic in 1982 found a high rate of concordance for hypodontia in MZ twin pairs

whereas the DZ twins that he observed were found to be discordant.

• These and the other previous studies conclude that the mode of inheritance could be

explained by a single autosomal dominant gene with incomplete penetrance.

Abnormal tooth shape:

• Alvesalo & Portin (1969) provided substantial evidence that missing and

malformed laterals may well be the result of a common gene defect.

• Abnormalities in the lateral incisor vary from peg shaped to microdontia

to missing teeth all of which have familial trends, female preponderance

& association with other dental anomalies such as missing teeth, ectopic

canines, transposition suggesting a polygenic etiology.

Ectopic maxillary canines:

• Various studies in the past have suggested a genetic tendency for ectopic maxillary

canines.

• Peck in 1994 concluded that palatally ectopic canines were an inherited trait, being one

of the anomalies in a complex of genetically related dental disturbances, often

occurring in combination with missing teeth, tooth size reduction, supernumerary tooth

and other ectopically positioned tooth.

CONCLUSION:

• A PERMANENT INTERACTION BETWEEN GENETIC AND ENVIRONMENTAL

FACTORS, BOTH OF A CONTINUALLY ALTERING NATURE, DETERMINE THE

DENTOFACIAL MORPHOLOGY.

• PREDOMINANCE OF MORPHOGENETIC PATTERN OF MALOCCLUSION

PROFOUNDLY INFLUENCES ORTHODONTIC OBJECTIVES AND THERAPEUTIC

RESULTS.

• THEREFORE MORE HEREDITARY STUDIES ARE IMPORTANT TO EVIDENCE

THE BASE OF GENETICS FOR THE PRACTICE OF ORTHODONTICS.

REFRENCES:• T. M. GRABER- ORTHODONTICS- PRINCIPLES AND PRACTICE

• T. C. WHITE, J. H. GARDINER, B. C. LEIGHTON- ORTHODONTICS FOR DENTAL STUDENTS.

• S GOWRI SHANKAR - TEXTBOOK OF ORTHODONTICS

• SRIDHAR PREMKUMAR –TEXTBOOK OF CRANIOFACIAL GROWTH.

• OMPRAKASH KHARBANDA – DIAGNOSIS AND MANAGEMENT OF MALOCCLUSION AND DENTOFACIAL DEFORMITIES

• THE USE OF TWINS IN DENTOFACIAL GENETIC RESEARCH . BY LAUWERYN I,CARELS C,VLIETINCK R (AJODO 1993;103:33-381)

• HERITABILITY OF CRANIOMETRIC & OCCLUSAL VARIABLES: A LONGITUDINAL SIB ANALYSIS. BY- EDWARD F HARRIS & MICHELLE

G.JOHNSON. (AJO-DO 1991;99:258-68)

• HERITABILITY MALOCCLUSION : PART 2. THE INFLUENCE OF GENETICS IN MALOCCLUSION. BY P.A. MOSEY; BRITISH JOURNAL OF

ORTHODONTICS/VOL 26/1999/195-203.

• HEREDITARY FACTORS IN TOOTH DIMENSION, A STUDY OF ANTERIOR TEETH OF TWINS. BY- HOROWITZ, OSBORNE AND DEGEORGE

(AJODO, APRIL, 1958/ VOL-28/ NO. 2)

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