DNA profiling is arguably the most powerful and discriminatory analytical technique available to the Forensic Scientist dealing with cases involving body fluids.
DNA Profiling General Discussion.
STR (Short Tandem Repeat) profiling is a form of DNA analysis. DNA is a very complex biochemical found in cells throughout the human body which carries genetic information ultimately determining the physical characteristics of a person.
That information is in coded form, half of which is inherited from each parent.
The DNA of any individual is the same in all of their body fluids and tissues and accordingly, the DNA recovered from a person’s blood will be the same as that found in their saliva, hair roots etc.
With the exception of identical twins, each person’s DNA is unique although, current technology does not allow the examination of every single difference between individuals. The techniques currently in use and applied to Forensic science looks at specific regions of DNA which are known to vary widely between individuals.
Current DNA profiling techniques can therefore be used to conclusively exclude someone as being the donor of an unknown source of DNA or, it may often provide very compelling evidence of association. Using the techniques, it is not possible to provide conclusive proof that a source of DNA had originated from a specific person.
STR profiling uses the technique of DNA amplification in which specific regions of DNA are targeted and copied many times, resulting in an increase in the amount of DNA available for analysis.
This means that an STR profile may be obtained from samples where the initial quantity or quality of DNA was unsuitable for other DNA profiling methods.
Until very recently (2014), the standard DNA profiling technique commonly in use was named SGM +, which amplifies (copies) 11 different regions of DNA, ten of which contain an STR. The remaining region, amelogenin, is used to determine the sex of the donor of the DNA.
Recent advances in DNA technology has resulted in a new profiling system, DNA-17, being developed, which analyses seventeen different areas of DNA, 16 of which contain an STR and the remaining area, which is known as amelogenin, indicates the sex of the donor.
Ten of the sixteen STR regions in DNA-17 are the same as the 10 regions in SGM+, and therefore the number of STR regions analysed has increased by 6.
The primary advantages of the new technology are increased sensitivity compared with SGM+, which is of assistance when very small amounts of DNA are recovered from a sample, and of course an increase in the number of regions of DNA that are analysed, which is of assistance in cases where only partial DNA profiles are obtained because the increase in the number of regions of DNA that are analysed implies that the obtained partial profile might contain additional information over and above that which might have been obtained from SGM+.
The methods of analyses in respect of the SGM+ and DNA-17 techniques are otherwise very similar.
The individual components of an STR profile are represented as a series of peaks which can be measured and assigned a numeric designation.
A person will have two peaks for each STR, one inherited from each parent, unless the same STR is inherited from both parents, when only one peak will be observed.
DNA evidence is variable and can range from a single source DNA profile to a very complex profile with a number of contributors, the latter often being difficult to interpret.
Our experts can undertake a thorough and robust evaluation of all aspects of DNA evidence that the Prosecution will seek to rely on including laboratory procedures, continuity of evidence, and the interpretation of multiple source, complex DNA mixtures