Molecular basis of inheritance, Patterns of genetic transmission, Gene mutation, structure of chromosome, chromosomes in Man, Genetic disorders, Numerical disorders, structural disorder, Genetics in an orthodontic perspective, Butler's field theory, methods of studying role of genes.
2. Introduction
• Uncertainty with which we crowd around a newborn baby trying to
establish whom he resembles putting into scrutiny each and every
part of his body.
• Genetics is the science concerned with the structure & function of all
genes in different organisms.
• 1814 Joseph Adams – founder of human genetics.
• Gregor Mendel – father of modern genetics.
3. Mendel’s Principles Of Inheritance
1. Fundamental theory of heredity:
- The passing of discreate units of inheritance, or genes, from
parents to offspring.
- Paired pea traits were either dominant or recessive.
- when pure-bred parent plants were cross-bred, dominant traits
were always seen in the progeny, whereas recessive traits were
hidden until the first generation (F1) hybrid plant left to self-
pollinate.
- Mendel counted the number of second- generation (F2) progeny
with dominant or recessive traits and found a 3:1 ratio of dominant
to recessive traits.
4. 2. Principle of segregation :
- During reproduction, the inherited factors (alleles) that
determine traits are separated into reproductive cells by a
process called meiosis and randomly reunite during fertilization.
- Separation occurs during meiosis hen the alleles of each gene
segregate into individual reproductive cells. ( eggs, sperm)
- When hybrid pea plants allow to self pollinate resulted in
progeny that looked different from their parents.
5. Molecular Basis Of Inheritance
1. The cell –
Basic unit of living body made up of different organelles, i.e. the cell wall,
cytoplasm, endoplasmic reticulum, ribosomes, mitochondria, nucleolus, etc.,
2. The chromosome –
Within the nucleolus of each cell are thread like structures of different lengths and
shapes called chromosomes.
3. The D.N.A.
Chromosomes are made up of long chains of Deoxyribonucleic acid (DNA)
molecules, twisted an twined in a specific manner.
6. 4. The gene –
- Basic unit of inheritance by determining the make up and structure o a
particular characteristic in an organism.
- A gene consists of a portion of the double stranded DNA molecule with 300 to
1000 nucleotide pairs.
5. Transcription –
- The process by which information is transmitted from DNA to the messenger
RNA at the initial stage of replication.
6. Translation –
- The process in which the genetic information is actually converted into protein
synthesis.
7. Patterns Of Genetic Transmission
1. Repetitive –
Recurrence of a dento- facial deviation within the immediate family and its
progenitors.
2. Discontinuous –
Recurrence of a malocclusion trait that reappears within the family background over
several generations but not continuously.
3. Variable –
The occurrence of different but related types of malocclusion within several
generations of the same family.
8. Gene mutations
• Protein synthesis for the process of replication is controlled by genes.
• A change induced by certain agents in the composition of the base pair of
the DNA molecule may lead to the synthesis of an altered protein.
• Gene mutation can be of different types like, visible mutations,
detrimental mutation, lethal mutation etc.,.
• Mutagens are agents that induce genetic mutations.
1. Ionizing radiation
2. Certain drugs, chemicals and food additives
3. Certain viruses
4. High temperature.
9. Chromosomes In Man
• 1956, Tjio & Lean, independently Ford & Hamerton demonstrated that
the number of chromosomes in man is 46 i.e. 2 pairs.
• 44 (22 pairs) are autosomes an a pair of sex chromosomes.
• The autosomes in the male & female cells are identical while the se
chromosomes are different.
• Males, out of 2 sex chromosomes, have one X and one Y chromosome
while females have 2X chromosomes.
10. Genetic disorders
• 2 types – Numerical disorders and Structural disorders
Numerical Disorders-
There is change in the number of chromosomes within the cell.
1. Polyploidy: A condition where there is an additional full set of chromosomes.
2. Monosomy: A condition where one autosome is missing.
3. Trisomy: A condition when there is an addition of a single chromosome only.
4. Klinefelter's Syndrome: sex chromosome abnormality in males where there
are additional X chromosomes.
5. Turner’s Syndrome: A sex chromosome abnormality in females with one X
chromosome missing.
11. Structural disorders-
• Structural disorders are those in which there is a change in the basic
composition and structure of the chromosome.
1. Translocation: An exchange of segments between non- homologous
chromosomes.
2. Deletions: Loss of a segment of the chromosome.
3. Ring chromosomes: deletion at both the ends of the chromosome.
Later the deleted ends stick together to form a ring.
12. Butler’s Field Theory
• According to this theory, mammalian dentition can be divided
into several developmental fields.
• The developmental field include the molar/ premolar field, the
canine and the incisor fields.
• Among the fields, dental variability manifests itself strongly in
the distal than in the mesial direction.
• E.g. The lateral incisor is more prone to variation than the
central incisor.
13. Methods Of Studying Role Of Genes.
• Twinning of human embryo seems to be natural’s answers to this heated
controversy.
• Human twins can be of 2 types:
• Monozygotic twins
• Dizygotic twins
• Monozygotic twins: they are two individuals developed from a single fertilized
ovum, which divides into two at an early stage of development. Monozygotic
twins thus have a genetic make-up identical to each other.
• Dizygotic twins: They are two individuals developed from two separate ova,
ovulated and fertilized at the same time. The two ova are fertilized by two
different sperms. They are not genetically identical as they develop from two
different embryos. They are analogous to siblings except that have an almost
similar developmental environment including the intra-uterine life.
14. Pedigree studies
• Definite trait of an individual is studied along his ‘ family tree’ so as to
find any hereditary influence.
• Many of the family traits like bimaxillary protrusion, missing teeth,
high arched palate etc., can readily be attributed to hereditary
inheritance.
• While doing these studies one should be aware of the dominant and
recessive traits and their expressions.
15. Inbreeding
• The mode of transmission of certain traits can be studied and their
dominant and recessive characteristics determined by analyzing
certain communities where practices like polygamy and ‘marriages
within the family ’ still exist.
16. Genetics - in an orthodontic perspective
• Micrognathia
• Macrognathia
• Cleft lip and palate
• Downs syndrome
• Gardners syndrome
• Marfan’s syndrome
• Cherubisum
• Cleido-cranial dysplasia
• Mandibulo-facial dysostosis
• Osteogenesis imperfecta
• Bimaxillary protrusion
• Bimaxillary atresia
• Retarded eruption of teeth
• Hypodontia, anodontia,
oligodontia, etc.,
• Abnormal overjet and overbite
• Open bite
• High arched palate
Dento- facial disturbances of genetic origin :-
17. • Relapse is not an uncommon phenomenon in orthodontics.
• while treating a patient orthodontically, we do change the genetic
expression of the patient.
• Once the treatment is completed and the appliance is removed, there is
always a chance that the genetic expression of the patient re-establishes.
• Ruling out a other possibilities like misdiagnosis, defective appliance
therapy and improper patient care, a great variety of relapse can possibly
be attributed to these genetic factors.
