Improving the Statistical Discrimination of mtDNA Analysis
Forensic science and its investigatory needs have been greatly improved by the success of mtDNA analysis. Whether it be solving a century old case, or one where nuclear DNA has lead to no avail, the value of mtDNA has provided investigators and researchers with the ability to compensate for the lack of results attained from any other form of evidence.
While the testing of mtDNA is much more advanced and reliable than that of nuclear DNA results, it still has its flaws and setbacks. One major discrepancy is that mtDNA can be classified into “types” which are somewhat common between certain races. Types can be defined as the sequences with “hypervariable” regions in the coding region of DNA. In this research, specifically, HV1/HV2 type that is most commonly found in about 7% of the Caucasian population was analyzed in 31 individuals.
The most important aspect of this research was handling the sequencing of the entire mtGenome. By utilizing this information, the 31 individuals with the common types were reduced to only 3 matching the profile. These areas that make this distinction are termed SNP (or single nucleotide polymorphisms) sites. By running the mtDNA against the entire mtGenome, it may be the most optimal technique in determining where these sites are in various populations of people.
Interestingly enough, the control region of DNA has a high evolutionary rate so the variation is about 10 times that of the gene-coding region. Therefore the variation values of the HV1/HV2 sites is quite concentrated. This should be cause for differentiation between people in the population. However, the coding region itself happens to be 15 times larger than the control. From this fact, it is evident that the largest amount of variation comes from the coding region.
Now, what does this mean in terms of the experiment? Since HV1/HV2 areas happen to be control regions, then it only makes since that so much of the Caucasian population shares a similarity in that aspect. In addition, there is still a chance for a random match to occur in the mtDNA analysis, but there are also many types that are quite rare in their own aspect which causes them to only be seen once in large databases. These databases have continued to grow exponentially and, therefore, the statical significance of a mtDNA type match can still be quite valuable.
With all this being said, why are the researchers so worried about these common types in the first place? I mean, if the match of a particular mtDNA type is so high, then why bothering sequencing the entire mtGenome to find further sites of proof? The problem lies simply in the types that are classified as common. The most troubling problem, according to the article is that, “the greatest limitation for mtDNA testing lies with the small number of common types for which the power of discrimination is low.”
This is where SNP sites come into play. The entire mtDNA genome is utilized to identify these sites in order to solve the problem of this low discrimination when the common types are being analyzed. Once these sites are identified, they are addressed by a few other factors to determine if they can be used in future analyses of common types. These factors include…
- Relatedness between the mtDNA types in question
- The evolutionary rate of the coding region sites that helped to lower discrimination
- The particular timing of the variation that resulted from mutation in the evolutionary history
Haplogroups can be defined as “clusters of closely related mtDNA lineages.” Due to the high level of relatedness, SNP sites are found to be the most successful in offering another source of discrimination, regardless of the type.
After comparison of the mtGenome of these types, it is evident that some of them only differ by 1 single base nucleotide. However, if these sites can be identified and proven to exist among multiple types, it could expedite the process and lead to an overall higher statistical value of any match made in the analysis itself.
Citations:
Parsons, Thomas J., and Michael D. Coble. “Increasing the Forensic Discrimination of Mitochondrial DNA Testing through Analysis of the Entire Mitochondrial DNA Genome.” Croatian Medical Journals 42.3 (2001): 304-09. Web.