Genetics in orthodontics

Genetics in orthodontics

Presented by RINCE MOHAMMEDFinal B.D.S

Genetics is the science concerned with the structure and function of all genes in different organism.

Founder of human genetics – Joseph Adams

Gregor Mendel – Father of modern genetics.

Why genetics in ortho? Growth, development and function of oral and

facial structure of an individual depends on his genes.

An orthodontist should know exactly why or how a malocclusion occurs, to what extend it express on next generation, what is its prevelance and how it react to the treatment plan and mostly if it can be prevented.

Genetics shed light on all these questions. It helps to segregate inherited malocclusions or aberrations of growth from those due to the effect of environmental factors.Thus help us to diagnose and prevent it from occurring in next generation.

Molecular basis of inheritence The cell - the basic unit of any living body. Chromosomes -thread like structures in the

nucleus. made up of DNA. D.N.A.- Each strand of DNA molecule is a chain of

nucleotides. Basic nucleotide unit comprise of deoxyribose sugar, a phosphate molecule and protein, either purine or pyramidine.

Gene – Gene forms the basic unit of inheritance by determining the make up and structure of particular characteristic in an organism.

Transcription -the process by which information is transmitted from the DNA to the messenger RNA.

Translation is the process by which the genetic information is actually converted to protein synthesis.

GENETIC DISORDERS - Its of 2 types

Numerical disorders - are those in which there is a change in the number of chromosomes within the cell. Normally it is 46.

Polyploidy – an additional set of chromosome is present.Monosomy - one autosome is missing.Trisomy - there is an addition of a single chromosome.Eg:Down syndrome -The person has 3 pairs of chromosome 21 rather than 2.Klinefelters syndrome – 22 A+ XXYTurner syndrome -22 A + OX -has only one sex chromosome.

Structural disorders-change in the basic composition and structure of chromasome.

Deletion – A portion of chromosome is missing / deleted.Duplication –A portion of chromosome is duplicated resulting in extra genetic material.Eg Charcot man tooth disease by duplication of chromosome 17.Translocation – When portion of one chromosome is transferred to another chromosome. Inversion - A portion of chromosome has broken off and turned upside down and reattained.There for the genetic material is inversed.

Mode of transmission of Malocclusions

Repetitive Traits :The recurrence of a single dentofacial deviation within the immediate family and in the progenitors. The same trait is seen generation after generation.

Discontinous Traits: It is the recurrence of a malocclusion that reappears within the family over several generations but not continuously.

Variable traits: the occurrence of different but related types of malocclusions within several generations.Eg: missing teeth - same teeth may not be missing in different generations.

Genetic influence in orthodontics.

• In 1836 Frederick G.kussel reported malocclusion both skeletal dental can be transmitted from one generations to other.

• Dento facial disturbances of genetic origin includes.

Micrognathia,MacrognathiaDown’s syndrome,Gardners syndrome,Marfan’s

syndrome,CherubismCleidocranial dysplasia,Ectodermal dysplasiaBimaxillary Protrusion,PrognathismOpen biteHigh arched palateHypodontia, AnodontiaHypoplasia and discoloration of teethFacial Clefts, cleft lip and cleft palate

A study of relapse on treated cases showed that relapse can be caused by hereditary factors.

Occlusal mannerisms,jaw positioning,and abnormal pressure habits are genetic in origin and may cause relapse.

Occlusal variations are determined by both genetic and environmental factors.

Butler’s Field Theory Butler divided the mammalian dentition

into several development fields. It includes molar/premolar field,canine

and incisor field Among the fields the maximum

variability manifests itself in the distal and the least in the mesial direction.Hence maximum variability will be seen for the third molars as compared to the first molars.More variability in lateral compared to central incisors.

Method of studying role of genes Twin studies Pedigree studies Inbreeding

Twin studies Done to evaluate the role of genetics played

in various traits. If both the individuals of the monozygotic

twins (they have identical genetic make up) express a trait that is seen in the parent,it is a strong evidence for a very significant role of genetics in that particular trait.

In case only one of the monozygotic twins express the trait and the other is absolutely normal, then we come to the conclusion that although there is a genetic component in the disease, there are some other factors which control the disease i.e multifactorial trait.

Depending upon the presence or absence of a particular trait twins can be classified into concordant and discordant twins.

If a particular character is seen in both the members they are said to be concordant.

if only one member of the pair expresses the trait they are said to be discordant.

Cleft studies suggest that the concordance rate in Monozygotic twins for cleft lip was 35% and that for cleft palate was 26%.

In Dizygotic twins the concordance rate for cleft lip was 5% and that for cleft palate was 6%.

This reflects the heritability of the condition; the higher the monozygotic concordance, the more important the genetic contribution and so the higher the heritability.

Twin studies reveales that Genetic variation has a major effect

on arch width and length. Identical twins were not occlusally

identical. Greater genetic basis for tooth size

and shape Known dimensions were largely

under genetic control

Pedigree studies-Most common Definite trait of an individual is

studied along his family tree so as to find any hereditory influence.

Inbreeding-The mode of transmission of certain traits can be studied and their dominant & recessive characteristics are determined by analyzing certain communities where practices like polygamy and marriages within the family still exist.

Contribution of genetics in various malocclusions

Class 2 & class 3 malocclusions have polygenic mode of inheritance.

Redman & Shapiro proposed that genes on X chromosome causes lengthening of mandible relative to maxilla.

High % of class 2-(shortened mandible) in XO subjects (Turner syndrome).

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 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.it includes Soft tissue (controls the position of upper & lower incisors),Digit sucking,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 Austrian 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.