Forensic DNA is the latest tool in criminal investigation. It is distinctly superior to other forensic tools like finger printing, ABO and HLA blood typing. DNA testing has achieved feats of crime detection unparalleled in the records of criminology.
Strengths DNA testing
It is much more discriminatory than other techniques. Complete blood group testing tells the odds of another having the same blood as the defendant as one in thousands and HLA typing says it is one in millions. Now compare the odds that DNA typing gives – one in billions.
DNA typing is very sensitive and is performed with DNA extracted from few hairs. Small samples can be amplified using polymerase chain reaction methods. This allows samples to be split into small amounts and sent to more than one laboratory for testing, thus reducing chances of errors.
Serologic testing requires blood but DNA testing requires only nucleated cells. DNA testing can therefore use, hair, urine, saliva, and other bodily fluids. DNA is more stable and long lasting than protein. While it does degrade with time it is less susceptible to environmental degradation than protein. DNA from skeletons has been used to identify dead soldiers.
Limitations of DNA testing
Earlier criticisms of DNA testing have been effectively countered by improved and standardized testing methods.
A valid criticism of DNA testing pertains to “Allele frequencies” used for identification. Allele frequencies differ between population sub-groups. A specific Allele under test may have a frequency of 4% among Asians and 1% among northern Europeans. But experts concluded that frequency of one in ten million or hundred millions made little difference. However, in principle this remains a valid limitation of DNA technique.
DNA Testing Techniques
DNA testing techniques show the difference in length between DNA sequences. Polymerase chain reactions (PCR) are used for the same purpose when samples are small or partly degraded.
There are two types of DNA fingerprinting, namely
Single locus DNA fingerprinting
This identifies polymorphism at a single locus and uses specific probes or PCR primers. From this method a DNA genotype is obtained. This method is used when DNA is degraded or the sample is mixed.
Multilocus DNA fingerprinting
This identifies simultaneously polymorphism at multiple loci. This is done by multiple single probes or a single probe, which identifies multiple similar sequence polymorphisms. In the latter case, a phenotype rather than a genotype is obtained. This method provides more information than the first one.
DNA typing matches sample from a site to that obtained from the suspect. Technically it does not determine if the sample comes from the suspect but rather gives the statistical probability of the sample not belonging to the suspect. A miniscule probability proves he is guilty. DNA testing has convicted and also acquitted suspects. In many cases acquittal was followed by conviction of the true culprit.
Forensic DNA testing has been successfully used in a variety of crimes. It has also been used to determine parentage and proving family relationships.