DNA Profiling and Forensic Odontology in Personal Identification

PART I: DNA PROFILING (DNA FINGERPRINTING)

Definition and Historical Background

• Developed by Dr. Alec Jeffreys in 1985
• First used to help solve two murder cases
• The probability that two individuals share the same DNA sequence is approximately one in a million billion (10¹⁵)
• Even between siblings, the chance of identical patterns is only one in 10,000 million
• With recent advances, even identical twins can be differentiated

Molecular Basis

• Each human somatic cell has 23 pairs of chromosomes - 23 derived from the father, 23 from the mother
• DNA is present only in nucleated cells
• The human genome contains approximately 6 × 10⁹ DNA molecules per diploid genome
• DNA is also found in mitochondria, and mitochondrial DNA (mtDNA) is exclusively inherited from the mother
• The double helix is stabilized by histone proteins (together forming chromatin)
• Base pairing: A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds)

Techniques of DNA Profiling

1. RFLP (Restriction Fragment Length Polymorphism)

1. DNA is extracted from biological sample and treated with restriction endonucleases, which cleave it at specific sites
2. The fragments are separated by agarose gel electrophoresis - smaller fragments migrate faster
3. The gel is stained with ethidium bromide and fluoresces under UV light
4. DNA is transferred from gel to nylon membrane by Southern blotting (capillary transfer technique)
5. Hybridization is performed - a single-stranded recombinant probe (tagged with P32 radioactive marker) is added; it seeks out its complementary sequence and hybridizes
6. Typically four probes are used sequentially, analyzing four separate DNA regions
7. After washing with 0.05% SDS, the membrane is placed against X-ray film to produce an autoradiograph

2. PCR (Polymerase Chain Reaction)

• A small amount of DNA is amplified more than a million-fold using thermal TAQ polymerase
• DNA is denatured by heating to >94°C, then small primer segments bind at the polymorphic regions of interest
• Temperature is raised to 72°C, causing TAQ polymerase to extend the primers and copy the strands
• The cycle is repeated 30 times or more, synthesizing over a million copies
• PCR can analyze 36 samples at a time
• Advantages: Simple, results within days, can analyze extremely tiny amounts, highly sensitive, easily automated, works on degraded material
• Disadvantages: Susceptible to contamination; PCR loci have fewer alleles than VNTR areas used in RFLP

3. STR (Short Tandem Repeat) Analysis

• Evaluates specific regions (loci) within nuclear DNA where short sequences (typically 4 base pairs) are repeated multiple times
• Different individuals have different numbers of repeat units, making each person's pattern unique
• Different STR-based DNA profiling systems are used in different countries
• The odds that two individuals will have the same 13-loci DNA profile is approximately one in one billion (10⁹)
• For forensic purposes, at least 13 core loci (the CODIS standard) must be studied
• Results can be detected by fluorescent detection or silver staining following electrophoresis

4. MLP vs. SLP Probes

5. VNTR (Variable Number Tandem Repeats)

• Uses probes that detect specific variable number tandem repeats
• Each VNTR probe recognizes one VNTR site (simpler banding than MLP)
• After establishing allele frequencies for each ethnic group, the probability of any particular pattern combination can be calculated

6. AFLP (Amplified Fragment Length Polymorphism)

• Relies on VNTR polymorphism; bands visualized by silver staining on polyacrylamide gel
• Popular in low-income countries due to low cost and ease of setup

7. Mitochondrial DNA (mtDNA) Analysis

• Many more copies of mtDNA per cell than nuclear DNA (1-2 copies of nuclear DNA vs. multiple mtDNA copies)
• Particularly useful for hair, bones, and teeth (which lack nucleated cellular material for STR/RFLP)
• All mothers share the same mtDNA as their daughters (strictly maternal inheritance)
• Comparing mtDNA of unidentified remains with a potential maternal relative is valuable in missing person investigations

8. Y-Chromosome Analysis

• The Y-chromosome passes directly from father to son
• Especially useful for tracing male lineages or analyzing biological evidence involving multiple contributors (e.g., gang rape)
• More rapid - can be done in 2 to 3 days
• Can be performed on small quantities of DNA
• Products from several STR loci can run simultaneously on one gel (as long as fragment sizes do not overlap)

9. Rapid DNA Technology

Medico-legal Importance of DNA Profiling

1. Criminal Investigation: Identification of perpetrators from biological evidence (blood, semen, hair, saliva) at crime scenes
2. Identification of Unknown Bodies: DNA profile compared (by single probe analysis) with parents, children, siblings, or other relatives from autopsy-derived tissues - for identification of missing persons, disaster victims, and decomposed/mutilated remains
3. Disputed Paternity and Maternity: Has become the gold standard, establishing parentage with near 100% accuracy - used in succession, maintenance claims, matrimonial disputes, criminal cases
4. Sexual Assault Cases: DNA of the victim can be recovered from penile swabs using STR DNA profiling up to 24 hours after intercourse (if the individual did not bathe)
5. Mass Disasters (bomb explosions, aircraft accidents): When other identification methods fail, DNA from tissues/bones/teeth provides definitive identification
6. Exoneration of the Innocent: DNA profiling can definitively exclude a suspect
7. Obliteration of Identity: When fingerprints, tattoo marks, and other features are deliberately destroyed, DNA profiling from residual tissues remains viable

PART II: FORENSIC ODONTOLOGY

Definition

Dental Identification

1. Restorative work
2. Unusual features
3. Comparison of antemortem with postmortem X-rays

Identifying Dental Characteristics (15 Key Features)

1. Number, spacing, and situation of teeth present: (a) unerupted and deciduous teeth, (b) permanent teeth (surface and configuration), (c) decayed teeth, (d) undersized/oversized teeth
2. Number and situation of absent teeth
3. Extraction sites: evidence of old or recent, healed or unhealed
4. General condition of teeth: (a) erosion, (b) cleanliness, (c) conservation, fillings and cavities, (d) color, (e) periodontitis
5. Peculiarities of arrangement: (a) prominence or reverse, (b) crowded or ectopic teeth, (c) overlapping, (d) malposition, (e) deformities, (f) rotation
6. Supernumerary teeth
7. Dentures: full, partial, upper or lower - type, shape, restorative materials used
8. Mesiodistal width of teeth
9. Any recognisable peculiarity of jaws, e.g., prognathism (prominence of lower jaw)
10. Old injury or disease; recently dislodged, loosened, chipped, or broken teeth
11. Special features: incisal edges, fractures, ridges, caries, etc.
12. Restoration and prostheses (surfaces, morphology, configuration, material)
13. Root canal therapy on X-ray examination
14. Bone pattern on X-ray examination
15. Oral pathology (tori, gingival hyperplasia)

Unique Dental Characteristics

• Most dentists believe that no two persons have identical dentitions
• Adult teeth have 160 surfaces - combining dental treatments, root formation, bone patterns, and tooth position, a person can be positively identified
• Conservative dental work or fillings is the most reliable identifier - fillings of various materials, root fillings, inlays, crown and bridge work
• In some individuals, the lower teeth protrude beyond the upper incisors (over-bite)

Special Tooth Findings

• Pink teeth: In decomposed or skeletonised remains, pink discoloration near the gum-line may be noted due to deposition of protoporphyrin. Asphyxia or CO poisoning as a cause has been disproved.
• Mottled teeth: Regional chronic dental fluorosis exhibiting an opalescent pattern on enamel - reflects environmental fluoride exposure during tooth calcification

Role of Radiography in Forensic Odontology

• Root shape
• Shape of pulpal canal
• Shape of fillings
• Abnormalities
• Bone trabeculation patterns
• Caries, tooth formation, and fractures

Medicolegal Importance of Forensic Odontology

1. Comparative Identification: Dental identification is the most sophisticated method of comparative identification after dactylography (fingerprinting) - when some features are available and records of a missing person exist. Unfortunately, it is of limited utility in developing countries where dentists often do not keep records.
2. Resistance to Destruction: Teeth and jaws are highly resistant to postmortem destruction:
   • Fire resistant (unlike fingerprints, which are destroyed by fire)
   • No changes with putrefaction (fingerprints are subject to putrefaction)
   • Resistant to decomposition even when soft tissues are gone
3. Mass Disasters: Dental identification is especially valuable in aircraft accidents, earthquakes, shipwrecks, bomb explosions, and house fires - identifications can be made from intact, mutilated, decomposed, skeletonised, or even burnt material
4. Age Estimation: Teeth are useful in estimating the age of an individual
5. Sex and Blood Group Determination: Sex and blood group can be determined from cells of the pulp cavity
6. Grievous Hurt: Loss of tooth due to assault is classified as grievous hurt (medico-legal significance in assault cases)
7. Disease Identification: Diseases such as caries, syphilis (Hutchinson's teeth) help in identification

Teeth vs. Fingerprints: Comparison

Bite Mark Analysis

• Usually semicircular or crescentic, caused by front teeth (incisors and canines)
• Gap at either side due to separation of upper and lower jaw
• May be abrasions, contusions, or lacerations
• In forcible bites: two "bows" with concavities facing each other, central petechial haemorrhages from sucking action
• Faint teeth marks become visible under ultraviolet light in a dark room

1. Photography: Photographs of bite marks are matched with tracings from positive dental casts. Transparent overlays or negative photographs of teeth are superimposed over positive photographs of the bite. Exclusion is generally easier than positive matching.
2. Casts: A rubber or silicone-based medium with catalytic hardener is laid over the bite mark to produce a permanent negative cast; plaster of paris, plasticine, or beeswax can also be used.
3. Digital Imaging: 3D datasets of bite marks enable high-precision measurements compared with suspect dentition measurements.

Tooth Charting Systems

• Palmer Notation (1870): Quadrant-based system using letters for deciduous (A-E) and numbers for permanent teeth (1-8)
• Haderup System (1902): Similar to Palmer's, uses + for upper teeth and - for lower
• FDI (Federation Dentaire Internationale) Two-Digit System: Two-digit number where the first digit denotes the quadrant (1-4 for permanent, 5-8 for deciduous) and the second denotes the tooth number

Interplay of DNA Profiling and Forensic Odontology

• Teeth as a DNA source: Teeth are one of the best biological specimens for DNA extraction in forensic settings. The enamel and dentin protect the pulp from environmental degradation, heat, and putrefaction - making dental DNA viable long after other tissues are unrecoverable. When antemortem dental records are absent, mtDNA or STR analysis from tooth pulp can still establish identity.
• In mass disasters, dental identification is used first for comparative identification when records are available; DNA profiling from dental pulp is used when records are absent.
• In skeletal and mummified remains, dental morphology establishes demographic parameters (age, sex, ancestry), while DNA from tooth pulp confirms specific identity.
• In mutilated bodies, the textbook recommends obtaining specimens for both toxicological analysis and DNA profiling - specifically "muscle, a piece of long bone, plucked hair" - alongside dental examination.