Nothing in forensics is simple
SCIENCE
In order for people to function effectively we need cells. As a particular cell grows and reproduces, it does this in an orderly fashion, faithfully following a blueprint and performing its numerous tasks with remarkable efficiency.
Many scientists (before and after Watson and Crick’s landmark discoveries of the structure of DNA) have contributed to a vast knowledge base of the DNA master-plan in our cells.
The practical application of these findings to forensics has allowed the legal fraternity to make use of this knowledge to indict or refute criminal allegations in court.
This has been done by looking at the probability that DNA markers at a crime scene match those of an alleged criminal. How does this differ from the practice of using fingerprints?
There are two major differences. One revolves around the statistics of DNA matching and the other relies on the sensitivity of the techniques involved, thus risking contamination. The risk of contamination leads to the potential for both real and arguable contamination arising from forensic sample collection by investigators in the case of DNA markers.
A fingerprint may be left by an intruder as a result of the person touching an object. The courts have relied heavily on the use of fingerprints to match criminals to a crime scene.
Certain points in a fingerprint pattern are used to ‘‘match’’ fingerprints. Fingerprints have generally been considered to be unique to an individual as no two patterns appear to be the same. DNA evidence has also been used to link alleged criminals to a crime scene.
However, the use of a ‘‘contamination’’ argument in legal defences, to instil sufficient doubt into the minds of deciding jurors and judges, appears more prevalent in the DNA evidence than with fingerprinting. This in turn provides alleged criminals with the potential to escape the consequence of their alleged offences, in the absence of other compelling evidence.
Both a strength and a weakness of DNA evidence arises from its remarkable sensitivity. The smallest trace of biological material left by a suspect can be synthetically amplified by replicating usable material, taken from a crime scene. DNA can be accurately copied and used for further profiling and matching analyses. This is different from finding a fingerprint (with its many millions of molecules left in the print trace).
Only a few DNA molecules are required for an entire analysis. If an object containing a small amount of material can conceivably be transferred by contact to any other material, it gives rise to the ‘‘contamination’’ charge being used as a legal argument. Even cottonwool swabs potentially could have been contaminated with DNA during their manufacture. This has resulted in legal considerations of this possibility in court.
The second major difference between DNA and fingerprinting technology, lies in use of statistics to link a probability of guilt with the statistical odds that a random person in the general population could be matched with the same DNA.
If 10 people in the population could have a similar profile this can be misunderstood as providing a 1 in 10 chance of guilt of the suspect. This is incorrect and is termed the ‘‘prosecutors paradox’’.
The evidence has to be weighed up with any other contextual evidence in the normal way that all evidence is considered. Generally the billions of cells in our bodies perform their function of replication nearly perfectly all the time.
In contrast, reliance, in the legal sense, on investigators performing these tasks perfectly, may allow for some doubt in terms of potential contamination risk and the use of statistics to support any findings. Nothing in forensics is as simple as it seems on television programmes.