Capillary electrophoresis is a qualitative analysis of the DNA collected. It utilizes electrical charge, molecular size and fluorescence and as the main working principles. DNA is a negatively charged molecule. With this property, it will tend to migrate towards a positively charged terminal since unlike poles attract (electroosmotic flow). In this method, there is a flow of gel that contains fluorescent-labeled DNA. This gel flows through a thin capillary tubing (~50um) that is made of glass. Along the tube, there is a detector that registers the amount of fluorescence emitted by the DNA in the gel. The larger the molecules the slower the movement through the tube. This whole process gives optimum results at temperatures of 600C. The fluorescent mission is plotted on an electropherogram and compared with that of an internal size standard that is labeled with a different color. The amount of fluorescence is measured in relative fluorescent units (RFUs). Therefore, their different allelic ladders are compared. This can, therefore, be used in comparing the sample collected with the sample of the suspects to enable matching.
Sex determination with AmelogeninDifferent cases require the identification of the sex of the involved individuals. Sexual assault is the one that mainly calls for this identification. Amelogenin assay is one of the methods used, and is, therefore, useful where there is a complex mixture of samples. This assay depends on the presence of the amelogenin gene that codes for enamel proteins. Amelogenin gene is found in both the X and Y-chromosome. The difference between the chromosomes is the presence of a 6-bp deletion in the X-chromosome that lacks in the Y-chromosome. PCR and electrophoresis can pick this difference. Potential errors can be noted in rare mutations mainly deletion or dropping out of the amelogenin locus.
Standard Tandem Repeats
These are the most common markers that help in the matching of DNA samples. These are short sequences repeated in a predictable fashion all over the genome. They can either be homozygous of heterozygous depending on the features of the alleles. The main distinctive factor that leads to individualization and determination of inheritance is their unique form of mutation. This occurs during replication when there is a slippage in the adjoining strip that thereby causes an unpaired portion of the strip to lead to a loss or gain of a repeat unit. Their mutation rate is quite high, and it typically affects multiple alleles. Due to this, it increases its discrimination as compared to other previously used markers. The 13 specific STR is the one mainly used by the Federal Bureau of Investigations (FBI) for evidence analysis since it has a 1 in a billion more chance of resemblance of DNA profile CITATION Nat12 \l 1033 (National Institute of Justice, 2012).
This is the DNA within the mitochondria. The ability to extract mtDNA from old non-nucleated cells gives it the upper hand in the determination of identity using hairs and even bone of remains collected in the field CITATION Nat12 \l 1033 (National Institute of Justice, 2012). The genome contained within the DNA is maternal. Therefore, all maternal relatives have the same mtDNA. This fact, therefore, rules out this method when determining paternity. This DNA contains a higher rate of mutation that the normal nuclear genes. Mitochondrial DNA shows a lot of geographic variation. Its main limitation is the power of discrimination. This is because, as compared to the CODIS STR panel that contains genes from both parents, it contains a single locus.
The non-coding D-loop is the main part of the mtDNA that is used in forensics. The hypervariable regions (HV1 and HV2) can be sequenced to determine heteroplasmy. Heteroplasmy is the presence of both wild-type and mutant genome within a cell or organelle. This feature is more common in the hair, and thus, caution should be taken to avoid misinterpretation. The main importance of this feature of mtDNA in forensics is that it can increase the level of probability of a match between samples thereby increasing surety regarding conviction.
Random match probability
In an ideal population currently, there is free movement of people; no restriction of mates; nor restricted number of offspring. Therefore, the occurrence of any allele depends on the fraction of the total population that it represents. The summation of all alleles is equal to one. Also the sum of all genotypes. Therefore due to the features of the current society that affect the population, we can see that the assumptions that guide the Hardy-Weinberg have to be modified to account thereby for the random allotment of the alleles that leads to linkage disequilibrium. Therefore, the methods have been devised to check therefore for the probability of the matching and involvement of a certain individual in relation to the acquired evidence. The lower the chance of getting a similar result of the result from any randomly picked person in the population increases the credibility of the evidence. This is called the likelihood ratio. The main function of this ratio is to increase the level of confidence in any conclusion reached.
National Institute of Justice. (2012, August 9th). DNA Evidence: Basics of Analyzing. Retrieved from National institute of Justice: http://nij.gov/topics/forensics/evidence/dna/basics/pages/analyzing.aspx
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