The field of forensic science which uses the study of teeth, their biology, properties, and structure in the application of solving crimes or in any procedure as such supporting the identification of individuals is referred to as forensic odontology. This field has been of great use especially in joint massacres when bodies are left in an unrecognizable state.
The characteristics of teeth, their alignment, and the overall structure of the mouth help in the identification of a specific person. Another application of forensic odontology in a criminal investigation is bite mark analysis.
A forensic odontologist can analyze the bite marks left on a victim and compare them with the tooth structures of a suspect to make a comparison. The use of dental records such as x-rays and dental casts allows dental remains to be connected to a victim particularly in circumstances such as decomposition, and odontological comparison is possible since the dental evidence often remains intact.
When no antemortem dental record is available for comparison, forensic DNA testing can be carried out for postmortem human identification. The mineralized dental structure protects DNA from degradation in cases where it may be degraded in other tissues of the body. Thus, teeth are an excellent source of DNA for forensic analysis under such conditions.
Different techniques of identifying individuals through dental means are available. Currently, there are four types of personal identification circumstances that use teeth, jaw, and orofacial characteristics which include comparative dental identification, reconstructive post-mortem, dental profiling, and DNA profiling.
DNA Analysis From Dental Remains
Extraction of DNA from the human body remains a difficult task and depends upon various environmental factors and extraction procedures. Most of the experts suggest that DNA extraction is more stable when it is from hard tissues like bone and teeth and after putrefaction of bodies.
Schwartz isolated HMW DNA from teeth at 4°C for up to 6 weeks. At 25°C, HMW DNA can be isolated after 19 years. At 37°C, teeth can yield HMW DNA following storage for 6 months. TC Boles could successfully extract DNA from teeth that had been buried for up to 80 years.
It is possible to discriminate one individual from all others with a high level of confidence by starting with only 1 mg or less of target DNA whereas, the amount of DNA that can be recovered from molar teeth with pulp volume of 0.023-0.031 cc is nearly 15-20 mg.
In various researches conducted, they were able to extract DNA from a dental sample from the total production of genomic DNA of about 6-50micrograms. The resultant DNA was from dental pulp and matched with the blood DNA collected from the same person.
Location of DNA From Dental Remains
The teeth differ in form and size having similar histological structures. The dentin is a connective tissue that forms the major structural axis of the tooth and is hardly exposed to the oral environment. The dentin on the crown of the tooth is covered by enamel. The enamel has an ectodermic origin and is an extremely mineralized tissue. Furthermore, it is an acellular and avascular structure without nerves. The root dentin is covered by cement, another type of calcified connective tissue.
Among all of these soft tissues within the coronal and radicular pulp chamber consisting of odontoblasts, fibroblasts, endothelial cells, peripheral nerve, undifferentiated mesenchymal cells, and nucleated components of blood, are all rich sources of DNA.
Some other anatomical joints or locations, that are less frequently used for the extraction of DNA include odontoblastic processes that extend into dentinal tubules, soft tissue within accessory canals, cellular cementum, adherent bone, and periodontal ligament fibers.
Recent studies have proven that one could obtain mitochondrial DNA (mtDNA) from dentine powder obtained through cryogenic grinding (an act of cooling and chilling a material to be later on powdered up). There are numerous methods for the isolation of DNA.
1. RFLP Technique
The substratum required for analysis is extracted from the specified DNA locations of the tooth and the DNA is digested and isolated which is further on, cut into fragments with the help of specialized restriction endonucleases enzymes. They act like molecular scissors for the process. They cleave the DNA at a specific site reorganizing a particular sequence.
The cut fragments contain the variable number of tandem repeats (VNTR) of varying lengths giving DNA fragments of different sizes.
The VNTR testing, which may present short repeated sequences of intermediate size, is rarely used in forensic analysis due to the poor-quality DNA provided with this method.
2. Polymerase Chain Reaction Technique
Polymerase Chain Reaction as we know amplifies the amount of DNA material available. In such rare and limited evidence as teeth, PCR is extremely important when it comes to DNA analysis.
To carry out the reaction, special enzymes and DNA primers are required. These primers are like probes with known constant sections of DNA but not labeled. They are designed to know constant sections of DNA at the ends of variable regions to be amplified.
The principle of PCR is that the DNA is capable of duplicating itself. This is done by unwinding the strands of DNA and each strand acts as a template for the synthesis of a new strand. By PCR technique we can amplify specific DNA segments dependent on the primer employed. The standard PCR reaction runs through 30 cycles in a couple of hours which results in amplification of original DNA by over 109 times.
The teeth are an excellent source of genomic and mtDNA because PCR analysis allows comparing the collected postmortem samples to known antemortem samples or parental DNA with the advantage of mtDNA providing a high number of copies per cell.
3. Y-Chromosome Analysis
The Y-chromosome is passed directly from father to son, so analysis of genetic markers on the Y-chromosome is useful for tracing relationships among males or for analyzing biological evidence involving multiple male contributors.
Since the beginning of the 90s, the field of forensic Y-chromosome analysis has been successfully developed to become commonplace in laboratories working in crime casework all over the world.
In Y-STR analysis, specific regions of DNA on the Y-male chromosome are targeted and copied many times. Y-STR DNA profiling system selectively targets male DNA even in the presence of large amounts of female DNA.
4. STR Analysis
In forensic samples, the study of DNA (genomic and mitochondrial) is usually performed by STR analysis, which can be defined as hypervariable regions of DNA that present consecutive repetitions of fragments that have 2-7 bp.
A forensic dentist can extract DNA from the pulp chamber to crossmatch and identify the victim. These dental records could be used by the investigators to match them to corpses. Similarly, ameloglyphics or enamel rod patterns could be studied and used further for investigations.
Like fingerprints, these are unique to each individual. Unlike fingerprints, they are highly resistant, and cannot be intentionally burned or cut to change the pattern. Similar to collecting fingerprints, investigators take enamel rod “prints”.
Other than these techniques, Single Nucleotide Polymorphisms(SNP) and Amplified Fragment Length Polymorphism(AFLP) is also used collectively in DNA analysis from dental remains.