Genetics in Orthodontics

Genetics
in
Orthodontics
Presented by -
Dr. Deeksha Bhanotia (PG-1ST year)
Under the guidance of –
Dr. Mridula Trehan (Professor & Head)
Department of Orthodontics &
Dentofacial Orthopaedics

Contents
• Introduction
• Common terms
• Cell
• Understanding DNA
• Chromosomes & Genes
• Inheritance
• Twin Studies
• Genetics and
Malocclusion
• Molecular genetics in
dental development
• Recent Studies
• Conclusion

Introduction
Genetics –
The branch of biology which deals primarily
with the principles of heredity & variation, &
secondarily with the role of environmental
factors as they interact with genes in the
development of an individual.
ORTHODONTICS: CURRENT PRINCIPLES & TECHNIQUES, 4TH EDITION, GRABER, VANARSDAL, VIG: 31-45

Heredity –
It can be defined as that force of nature which
permits the transmission of characteristics of a
species from generation to generation.

Basic terminologies
• Allele- alternative forms of a gene found at
same locus on homologous chromosomes.
• Autosome- is any chromosome other than
the sex chromosome.
• Gamete - a germ cell (sperm or ovum )
containing haploid number of chromosomes .

• Gene- is a part of DNA molecule which
directs the synthesis of specific polypeptide
chain.
• Genotype- genetic constitution of an
individual.
• Phenotype -it is the final product or all the
observable characteristics of an individual.

• Trait - any detectable phenotypic property
or character.
• Linkage - two genes situated close together
on same chromosome are said to be linked.
• Chromosomes - thread like deep staining
bodies situated within nucleus.
• Chromatin - it is the name given to the
material of which chromosomes are made.

Development of genetics
• Mendelism - Story of genetics really starts
with the work of Gregor Johann Mendel, a
monk in Moravia. He was born on July 22,
1822.
He made his discoveries by analysis of
results after crossing varieties of Garden
pea (Pisum sativum)
THE HERITABILITY OF MALOCCLUSION: PART 1—GENETICS, PRINCIPLES AND TERMINOLOGY, BRITISH JOURNAL OF ORTHODONTICS, VOL. 26, NO. 2, 103-113, JUNE 1999

• He took – round or wrinkled seeds
tall or dwarf plant
violet or white flowers
• He crossed varieties differing only in one pair of
these characteristics.

In his monohybrid experiments he found
the genotypic ratio of 1 when he crossed
pure varieties with opposing traits.
R R
r Rr Rr
r Rr Rr

R r
R RR Rr
r Rr rr
Crossing the second generation in which he
obtained plants with only round seeds , he
obtained genotypic Ratio of 1:2:1

Dihybrid cross:
When he crossed the second generation of plant
variable in two characteristics he obtained a
phenotypic ratio of 9:3:3:1 , and among the 9
smooth yellow obtained genotypic ratio was
1:2:2:4.

SY Sy sY sy
SY SSYY SSYy SsYY SsYy
Sy SSYy SSyy SsYy Ssyy
sY SsYY SsYy ssYY ssYy
sy SsYy Ssyy ssYy ssyy

Mendel’s laws:
• Law of Uniformity –when 2 homozygotes with
different alleles are crossed all of the off springs
of first generation are identical heterozygotes.

• Law of Segregation –each individual possess
2 factors which determine specific
characteristic, a parent transmits only 1 of
these factors to any particular off spring. It is
purely a matter of chance which of the 2
factors get transmitted.

• Law of Independent assortment –
independent assortment occurs only when
genes affecting different characters are on
different chromosomes. In other words
genes that are not alleles are distributed to
gametes independently to each other.

Cell Structure & Division
Cell
cytoplasm
nucleus
cell membrane
cell organelles

Cell Structure

18
hours
G 1
S
G 2
M
Cell Cycle

Cell division
Mitosis
Meiosis
In mitosis a cell gives rise to 2 daughter cells
which are identical to the parent cell, in this
the daughter cell receives the complete
chromosome set as the parent

• In meiosis there is reduction in the number of
chromosomes to half, this process is carried out
in gametogenesis ,it is further divided into
meiosis I & meiosis II

Mitosis
• Prophase –centrioles separate, chromosomes
become distinct ,nuclear membrane breaks
• Metaphase – arrange at equator ,attach to
spindles
• Anaphase – centromere splits
• Telophase- 2 daughter cells formed ,nuclear
membrane reappears

Meiosis I
• Prophase I- Leptotene
- Zygotene
- Pachytene
- Diplotene
- Diakinesis
• Metaphase I
• Anaphase I
• Telophase I

• Leptotene- chromosomes Ist appear as long,
slender threads with many bead like structures
(chromomeres) along their length
• Zygotene –homologous chromosomes appear to
attract each other & enter into a very close zipper
like pairing (synapsis),pairing is highly specific

• Pachytene – progressive shortenings &coiling of
chromosomes. Crossing over of homologous
regions of the bivalent chromosomes / tetrad
chromatids occurs. (exchange of genetic material
between non-sister homologous chromatids
takes place)

• Diplotene – distinctly visible separation occurs
between homologous chromosomes except for
specific regions where an actual crossing over has
taken place ,these crossed areas (chiasmata) are
X shaped attachments between the
chromosomes & are the only remaining force
holding each bivalent together until metaphase

• Diakinesis – coiling &contraction continues ,
bivalents migrate close to nuclear membrane ,
nucleolus disappears

• Metaphase I-nuclear membrane disappears
• Anaphase I- one entire chromosome of the
pair move to either pole
• Telophase I- membrane reappears

•After meiosis I meiosis II continues which is
exactly like mitosis .
Differences
• Mitosis
Takes place in somatic
cells
Two daughter cells are
formed
• Meiosis
Takes place in gametes
Four daughter cells are
formed

• Each cell formed has
diploid chromosomes
• Single step procedure
• In anaphase
centromere cleaves,
sister chromatids are
cleaved
• Each cell has haploid
chromosomes
• Occurs in two parts I
& II
• In anaphase I there is
no cleavage of
centromere, sister
chromatids are
cleaved in anaphase II

Understanding DNA
• Human body consists
6000 billion cells
constituting various
tissue & organ systems
• DNA is considered as
chemical blue print of
life

• DNA –Deoxy ribonucleic acid
• Consists of a sugar, phosphate ,&nitrogen base
• Nucleotide = P +nucleoside
• Nucleoside = nitrogen base + sugar

DNA consists of :
**Deoxyribose sugar(5C)
**A phosphate group
** Nitrogen bases - that are further divided into
* purines –adenine ,guanine
* pyrimidines –thymine, cytosine &
uracil(present in RNA in place of thymine)

• It is a double helix which consists of phosphate-
sugar back bone
• Back bone is formed by phosphodiester bonds
between the 3’& 5’ carbons of adjacent sugars
• Bridging the gap between the two chains are of
paired nucleotide bases

• The chains are anti parallel to each other
• Adenine & thymine are connected by double
hydrogen bond

• Guanine &cytosine are connected by triple
hydrogen bond
• A=T &G=C in other words A+G=T+C
(purines=pyrimidines ) “Chargaff’s rule”
Replication : process of copying of DNA
2 chains of DNA separate, each chain builds up its
complement, giving rise to 2 identical daughter
DNA

• DNA has genes & hyper
variable regions(hvr) in
the human genome
• hvr are unique to each
individual

• Single DNA molecule comprises of 50-500 million
base pairs
• Total number of genes in a human genome are
said to be 10000 ,it comprises of only 2-5% of
total body DNA

• The function of remaining 95% DNA is not
known, such non coding DNA is called as junk
DNA
• This DNA contains regions where same sequence
repeat themselves over & over like a stutter
(repetitive DNA)
• These regions containing repetitive DNA
demonstrate hyper variability from person to
person

• There is an extremely degree of variability in such
tandemly repeated DNA & these variants are
called “variable number tandem repeats” (VNTR)
• SuchVNTR is studied in DNA fingerprinting
• There are more than 1500VNTR in human
genome

Human chromosome
• These are thread like structures located in the cell
nucleus
• Each species has a specific number of
chromosomes
• Humans have 23 pairs or 46

• 22 pairs are autosomes ,1 pair is sex
chromosomes XX/XY
• Chromosomes vary in shape depending on the
position of centromere

• Individual chromosomes differ not only in
position of centromere but also in their
overall length(amount of DNA )
• Human chromosomes are divided into 8
groups depending on size ,position of
centromere, presence or absence of satellites

• If the allele are identical it is called homozygous ,
if the alleles differ it is called heterozygous

Centromere
Middle
Off center
If close to the end
At one terminal end
tip
Name
Metacentric
Sub metacentric
Acrocentric(in this
the short knob like
on the short arm is
called satellite
Telocentric

Group chromosome description
A 1-3 Largest ;1&3 mc,2sc
B 4-5 Large ;sc
C 6-12 &X Medium size;sc
D 13-15 Medium size;ac with
satellites
E 16-18 Small ;16 mc,17&18
sc
F 19-20 Small;sc
G 21-22 &Y Small;ac

Chromosomal Nomenclature
1. p short arm
2. q long arm
3. Cen centromere
4. t translocation
5. inv inversion
6. del deletion
7. / mosaicism
8. +/- sign in front –whole chromosome
is added or missing
9. +/- sign follows the chromosome –piece of
chromosome is missing or added

• If a trait or disease manifests itself when only
one gene is present along with a normal allele
the trait is called dominant
• If trait manifests itself when both alleles are
alike it is called recessive

• In X linked traits male to male transmission is
impossible
• In X linked recessive traits only female carriers
are found, and male manifests the trait as male
has only 1 X chromosome

Genes
Types
i. Structural genes
ii. Control genes
iii. Pseudogenes
iv. Housekeeping genes

Operon = operator gene +structural gene
Operon is controlled by regulator gene which is
not necessarily close to the operator gene

Genetic code (triplet code)
• Genetic information is stored in the DNA
molecule in the form of triplet code which refers
to series of three bases in DNA or RNA which
codes for specific amino acids

• Codon –triplet of nucleotide bases in m-RNA
which codes for a particular amino acid
• Anticodon –complimentary triplet of t-RNA
molecule which binds to m-RNA codon

• Genetic code is universal
• Codon’s are non overlapping
• It is degenerate
DNA is formed in the nucleus but proteins are
formed in cytoplasm ,this involves two processes
1. Transcription DNA RNA
2. Translation RNA protein

Central Dogma
DNA
RNA protein

Mutations
A mutation is defined as an alteration or change
in genetic material
Mutations are due to
• Mutagenic factors –radiation, chemicals etc.
• Spontaneous errors in DNA replication &repair

Mutations can be coded or noncoded ,
Coded are the ones that are transmitted
Mutations of the somatic cells cannot be
transmitted ,if it occurs in gamete cells it can be
transmitted
Mutations can be further divided into ;
length mutations & point mutations

• In a point mutation a single nucleotide base is
replaced by a different nucleotide base
transition –purine to purine/pyrimidine to
pyrimidine
transversion – purine to pyrimidine or vice
versa
• Point mutations can be fixed/stable or dynamic /
unstable

• In absence of mutagenic factors the mutation
rate is in the order of one base pair substitution
for every billion base pairs replicated
• Synonymous –mutations does not alter the
polypeptide product of gene also called silent
• Non synonymous –mutations lead to alterations
in the encoded polypeptide

• Missense –a simple base pair substitution can
result in coding for a different amino-acid and
synthesis of altered protein
• Nonsense –a substitution that leads to the
generation of out of stop codon which will result
in premature termination of translation of
polypeptide chain

• Deletion/insertion: deletion is the loss of 1 or
more nucleotides ,insertion is the addition of 1 or
more nucleotides –results in FRAME SHIFT
mutation if these are not in a multiple of 3

• The majority of mutations are likely to cause
reduced fitness ,a reduced ability of the resulting
zygote to contribute progeny to next generation ,
in this way harmful genes tend be eliminated
from the population

• A balance between the production of
disadvantageous alleles through mutation &their
elimination by selection results in the presence of
harmful alleles in the population at a low
frequency

Chromosomal aberrations
• Alterations of the genetic material which
involves many genes & large amount of DNA
1. Numerical aberrations
2. Structural aberrations
THE HERITABILITY OF MALOCCLUSION: PART 2. THE INFLUENCE OF GENETICS IN MALOCCLUSION, BRITISH JOURNAL OF ORTHODONTICS, VOL. 26, NO. 3, 195-203, SEPTEMBER 1999

1. Numerical aberrations
• Euploidy – triploidy, tetraploidy
• Aneuploidy – monosomies, trisomies
tetrasomies

2. Structural aberrations
• Translocations –transfer of genetic material
from one chromosome to another(non
homologous)
• Inversions –rearrangement within the same
chromosome ,segment is rotated 180
degrees

• Insertion –one segment is removed from
normal position &inserted in different
position
• Deletion –a missing chromosomal segment

Chromosome
abnormality
syndrome C/F
13 trisomy Patau’s Mental retardation
Microcephaly
Cleft lip/palate
Micrognathia,small
eyes
18 trisomy Edward’s Mental retardation
Brachycephaly
Micrognathia
Hypodontia
CLP
XO Turners (exception) Retarded growth
Micrognathia
Spade like chest
Ovarian agenesis

21 trisomy Down’s Brachycephaly
Mental retardation
Max hypoplasia
Flat nasal bridge
Delayed eruption
Growth retardation
Moon face
macroglossia
4p- Wolf-hirchhorn Mental retardation
Abnormal facies
CLP
5p- Cri –du-chat Mental retardation
Microcephaly
Characterstic cry
XXY Klinefelter’s Gynecomastia
Small testes
Decreased facial hair
Eunuchoid habits

Modes of inheritance:
• Medelian inheritance
• Non-Mendelian inheritance
• Polygenic inheritance

Mendelian inheritance /
single gene inheritance
Over 800 traits in human exhibit mendelian
inheritance e.g. blood group, hemophilia
Autosomal inheritance and sex linked
inheritance both can be dominant or
recessive

In autosomal dominant inheritance both the
sexes are equally affected and presence of only
one dominant allele manifests the trait.
In autosomal recessive traits both the alleles
(homozygous) should be present

• X Linked Dominant inheritance – both males and
females are affected but females are more
frequently affected
• Affected male transmit the trait to his
daughters but not the son
• Expression is less severe in female
heterozygotes than affected males

• X Linked Recessive inheritance – this trait affects
males ( because of only 1 X chromosome )
Females are usually carriers

• Y Linked Inheritance / Holandric inheritance –Y
chromosomes contains only few genes
About 20 –25Y linked genes have been identified
e.g. hairy ears, webbed toes

Non Mendelian inheritance
A number of disorders are known which do not
follow patterns of Mendelian inheritance several
mechanisms account for this :
• Anticipation
• Mosaicism

• Uniparental disomy
• Genomic imprinting
• Mitochondrial inheritance

• Anticipation: the disease occurs with
increasing severity in subsequent generation
• Mosaicism: occasionally a chromosome
change occurs after the zygote has been
formed which may lead to sections of tissues
growing side by side bearing different
chromosomal constitution such individuals
are known as Mosaics/ Chimeras

• Uniparental disomy: individuals who inherit
both homologous chromosomes only from
one parent

• Genomic imprinting: Although it was originally
believed that genes on homologous
chromosomes were expressed fully, it is now
recognized that different clinical features can
result depending on whether a gene is
inherited from father or mother

• Mitochondrial inheritance: mitochondria and
its DNA are almost exclusively inherited from
mother through oocyte e.g. diabetes with
deafness

Many traits having strong genetic component are
• Height
• Intelligence
• Birth weight
• Diabetes mellitus

• Schizophrenia
• Hypertension
• Cleft lip &palate

Polygenic inheritance
• Many of these traits do not follow the simple
mendelian genetics, but are dependent on
several genes i.e. are determined by a
constellation of several genes ,some derived
from each parent

• Determination of heritability for polygenic
characters is difficult e.g. mandibular
micrognathia can occur in chromosomal
disorders such as turner’s syndrome ,in
monogenic disorders such as treacher collins
syndrome or non syndromic polygenic factors

• Discontinuous polygenic traits
• Continuous polygenic traits
• Etiologic heterogeneity

Discontinuous polygenic traits
• When present these traits can vary continuously
• There is an underlying scale of continuous
variation of liability to develop the condition
resulting from a combination of all the genetic &
environmental influences involved

• The condition occurs only when the liability
exceeds a critical threshold value & the greater
the value beyond the threshold the more
severe the disease

• Cleft lip & palate is a congenital malformation
inherited this way
• The parents of the affected are often unaffected
& there may be no family history
• Giving birth to an affected child shows that
parents have some underactive genes for lip &
palate formation

• Some multifactorial traits show unequal sex
ratio ; like more common in males ;the
incidence is increased in relatives of affected
males & is even more increased in relatives of
affected females
• This indicates that for this malformation the
female threshold is higher than the male
threshold

Continuous polygenic traits
• Many normal human characteristics come
under this type
• Height
• Malocclusion

Etiologic heterogeneity
Both continuous & discontinuous variation have a
multifactorial basis so that different patients are
not necessarily affected for same reason

• Cleft palate patients no single cause can be
identified ,it can be due to chromosomal disorder
–Wolf Hirschhorn syndrome, trisomy 13 (patau
syndrome),in monogenic disorders such as
VanderWoude syndrome ,it may also be
associated with cigarette smoke ,alcohol , drugs

• Linkage-it refers to closeness of two genes in any
of their allelic forms on the same chromosome
• Penetrance –it is a statistical term &indicates the
proportion of individuals carrying a certain gene
who can be detected
• Expressivity- it refers to degree of expression of
a gene in an individual

Twin studies
• Twinning –when birth is given to 2 infants at the
same time they are called twins
• Twin studies provide an efficient way of studying
the influence of heredity & environment (nature –
nurture)

Twins
Monozygotic Dizygotic

• Dizygotic /fraternal
twins –they develop
from two different
embryos
• Genetically alike like
any other siblings
• Can be of different
sex
• Resemblance only
like siblings
• Monozygotic
/identical twins –they
arise from a single
fertilized ovum
• Identical genetic
makeup
• Same sex
• Resemble each other

Basic methodology
• Providing the zygosity of twins whether they are
monozygotic or dizygotic
• Studying the effect of heredity on craniofacial
development among monozygotic & dizygotic
twins & comparing development among the two
types &comparing them to find out the
differences

Various methods have been used to differentiate
• Hair & eye color
• Ear form
• Dermatoglyphics
• Teeth morphology
• Phenylthiocarbamide taste sensitivity
• Blood groups
• Serum proteins (gamma globulins)

• Concordance- when both members of a pair of
twins exhibit same trait
• Discordance –if only one has the trait
Principle
• MZ twins have identical genotype ,any
phenotypic difference environment is responsible
• Difference in DZ twins are both genetic &
environmental

Twin studies for occlusal & dentofacial
structures
1. Lundstorm (1948)- concluded that genetic
factors have greater influence on craniofacial
structures
2. Lundstorm (1955)- genetic factors have greater
influence on sagittal apical base relationship

Krause Wise & Frei (1959)- concluded that
morphology of craniofacial bones are under strong
genetic influence
Krupanidhi &V. Surendra Shetty(1989)- assessed
amount of genetic & environmental influence on
dental measurements in twins

• Statistically insignificant intrapair difference in
MZ twins in 11 out of 13 parameters (PMBAW,
PMD, basal arch width, tooth material, arch
perimeter, inter-canine width, overjet, overbite,
curve of spee, midline)

• Intermolar width & palatal depth showed
significant difference
• significant difference in DZ twins for 5
parameters showing difference may be due to
different genetic material

• Ferguson –Smith(1993) cleft study ,the
monozygotic twin concordance rate for CLP & CP
was 35 & 26 % respectively
dizygotic twins 5& 6 % respectively

Disorders of tooth development
• Control of tooth development –homeobox genes
have particular implication in tooth development
• Msx 1 & Msx 2 appear to be involved in epithelial –
mesenchymal interactions &are implicated in
craniofacial development

• Msx 1 & Pax 9 have been implicated in non
syndromic tooth agenesis
• Bone morphogenic proteins (BMP) have role in
dentinogenesis other than inducing osteogenesis

Amelogenesis imperfecta
• It is a group of genetically heterogeneous
disorders affecting enamel formation
• Hypoplastic, Hypocalcified, & Hypomaturation
forms have been described (Witkop 1988)

• It exhibits autosomal dominant, autosomal
recessive & X-linked inheritance
• In humans two amelogens, AMGX & AMGY have
been cloned &mapped to X &Y chromosomes

• In 1997 Mc Doughall et al mapped the
ameloblastin gene within critical region for
autosomal dominant AI at chromosome 4q21

Dentinogenesis imperfecta
• It is autosomal dominant
• Occurs in about 1:8000 live birth
• Presents with brownish discoloration of teeth ,
crowns susceptible to rapid attrition, fragile roots
& obliterated pulp chamber

• One type of DI is coupled with osteogenesis
imperfecta
• Most patients with this type of DI have
mutations &deletions for amino acid
substitutions in genes for type 1 collagen

Hypodontia
• Msx 1 is strongly expressed in dental
mesenchyme throughout the bud, cap &
bell stages of odontogenesis

• Vastardis et al (1996) demonstrated that a
mutation in Msx 1 caused familial tooth agenesis
& genetic linkage analysis of a family with
autosomal dominant agenesis of 2nd premolar &
3rd molar identified a locus on chromosome 4p as
the site of the Msx 1 gene

Ectodermal dysplasia
• Hypohidrotic ectodermal dysplasia is a
heterogeneous disorder with clinically many
distinct types
• It is characterized by triad of –
hypotrichosis
hypohydrosis
hypodontia

• Hypodontia varies from few missing teeth to
complete anodontia
• Kere et al identified the gene for X-linked EDD

• The form & size of teeth is principally determined
by genetic factors ,growth of the dentofacial
structures is undoubtedly influenced by
environmental as well as genetic factors

• The influence of genetics on dentofacial
morphology does not imply that genetic
information is solely located in bones ,but also
in neurological ,muscular &neuromuscular fields
which have influence on skeleton

• Mastication, speech, facial expression &
swallowing are examples of neuromuscular
patterns, although they are under conscious
control, there is no basis to suggest that these
patterns can be changed permanently

• Although the bones are also influenced by
functional matrix, the functional matrix
comprises of neuromuscular activity which is
influenced by genetics as well as environmental
factors
• This will have a direct bearing upon the extent to
which a particular malocclusion can be influenced
by therapeutic environmental intervention

• Salzman (1972) highlighted the familial nature of
tongue thrusting, jaw posturing, & orofacial soft
tissue mannerism

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.
Genetics and Malocclusion

• A higher correlation between the patient and the
immediate 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.

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.

Class III
• One of the famous examples was the
mandibular prognathism that was running in the
Australian monarchy -The Hapsburg jaw.
• Strohmayer in 1937 concluded from detailed
pedigree analysis that the mandibular
prognathism was transmitted as an autosomal
dominant trait.

• 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.

Recent Advances
Homeobox genes
• Homeobox genes are genes, which are
highly conserved throughout 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 the Hox group, Msx 1 and
Msx 2 (muscle segment), Dlx ( distalless ), Otx (
orthodontical ), Shh ( sonic hedgehog ).
• The proteins encoded by the homeobox genes
are transcription factors, which control
transcription of RNA from DNA template within
the cell nucleus.

• At the cellular level this control is expressed
through two main groups of regulatory
proteins, the growth factor family and the
steroid/thyroid/retinoic acid super family.

Genetic influence on tooth number, size,
morphology, position & eruption
• Msx 1 &Msx 2 are responsible for stability in
dental patterning, this is in confirmation with
Butler’s field theory
• Clinical evidence suggests that congenital
absence of teeth &reduction in tooth size are
associated

• A study of children with missing teeth found up
to half of their siblings or parents also had
missing teeth

Supernumerary teeth
• Brook (1974) reported that prevalence of
supernumerary teeth in British school children
was 2.1% in permanent dentition with
male:female ratio of 2:1 ,in Hong Kong the
prevalence was around 3 % & male :female ratio
6.5:1

• most common supernumerary tooth is the
mesiodens, these are commonly present in
siblings &parents of the patients, it does not
follow simple mendelian pattern

Abnormal tooth shape
• Alvesalo & Portin (1969) provided substantial
evidence supporting the view that missing ,
malformed lateral incisors may be the result of
common gene defect ,all of these defects show
familial trends, female preponderance &
association with other dental anomalies ,such as
other missing teeth ,ectopic canines ,suggesting a
polygenic etiology

Ectopic maxillary canines
• Peck et al (1994) concluded that palatally ectopic
canines were an inherited trait ,being one of the
anomalies in a complex of genetically related
dental disturbances –supernumerary teeth ,
missing teeth ,transposition ,tooth size reduction,
other ectopically positioned teeth, it seems to be
a polygenic trait. In European population PDC is
2-3 times more common than LDC.

Submerged primary molars
• It occurs most commonly in mandibular arch ,
there is a high rate of concordance between the
monozygotic twins ,number of studies provide
evidence for genetically determined primary
failure of eruption

Maxillary midline diastema
• Numerous etiologies have been proposed –
toothsize arch length discrepancy ,aberrant labial
frenum attachment ,parafunctional habits, tooth
loss ,periodontal disease, deep bite ,maxillary
midline pathology , Broadbent phenomenon ,
Gardinger stated that parents & offsprings appear
to share dental phenotype ,a few authors found
heredity to play a greater role in MMD

Conclusion
 Due to limited knowledge about the genetic
mechanisms involved and the precise nature and
effects of environmental influences, we are
unable to predict with a satisfactory degree of
certainty in the final manifestation of the growth
pattern or the severity of the malocclusion
conferred by a particular genotype.

• It is therefore necessary for the orthodontist to
know the influence of genetics on growth & the
development of malocclusion.

References
* ORTHODONTICS: CURRENT PRINCIPLES & TECHNIQUES, 4TH EDITION,
GRABER, VANARSDAL, VIG: 31-45
* THE HERITABILITY OF MALOCCLUSION: PART 1—GENETICS, PRINCIPLES
AND TERMINOLOGY, BRITISH JOURNAL OF ORTHODONTICS, VOL. 26, NO.
2, 103-113, JUNE 1999
* THE HERITABILITY OF MALOCCLUSION: PART 2. THE INFLUENCE OF
GENETICS IN MALOCCLUSION, BRITISH JOURNAL OF ORTHODONTICS,
VOL. 26, NO. 3, 195-203, SEPTEMBER 1999
* CLASS II DIV II MALOCCLUSION; A HERITABLE PATTERN OF SMALL TEETH
IN WELL DEVELOPED JAWS, ANGLE ORTHODONTISTS VOL 68 FEB 1997

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