Is DNA Evidence Enough to Secure a Guilty Conviction?
DNA evidence of an individual’s match to a crime scene, is often felt to indicate complete certainty of guilt. Theoretically, only one person in each billion has a particular DNA sequence. From this, it is assumed that if a technique matches two DNA sets one on the individual and the other at the crime scene this must prove guilt. However, in each case, like the current arrest of Dennis Earl Bradford for the assault of Jenniffer Schuett, 20 years ago in Dickinson, Texas, this evidience is still only circumstantial. It is important for any investigation and subsequently the jury to consider all reasons why there could be a DNA match other than an assumption guilt.
Horrific Crimes and Assumptions of Guilt
When there has been a horrific crime inevitably lead to a desire for justice. , The details about the assault of Jennifer Schuett, make the stomach turn with revulsion and the pent up longing for fairness is exacerbated by the 20 year delay in bringing the perpetrator to trial. This natural desire and revulsion must be tempered by a cool head and objective view of evidence. It is no justice to convict the wrong person, as this provides the guilty with even more protection. When DNA evidence comes to the aid of the prosecutor their are some very tempting numbers to thrown around, after all each individual shares their DNA sequence, “finger print” with only 6 others in the whole world, roughly one in a billion.
Unique DNA Profiles and Plausible DNA Matching
So the argument goes, if I can demonstrate that the same DNA profile is obtained from the alleged assailant, Dennis Bradford, as is found on items from the crime scene, the victim’s clothes, or most conclusively the victim’s body, this must show that there is a one in a billion chance of the guilty man not being Dennis Bradford. Surely, one in a billion is beyond all reasonable doubt, so the case is closed, the guilty ceased and sentence passed.
The Prosecutor’s Fallacy and the Assumption of a Random Selection.
However, this argument has a common and unfortunate legacy of miscarriages of justice, it has gained the name the “prosecutors fallacy”. The probability of a getting a practical DNA match between the accused and the crime scene is a vastly different number to the theoretical likelihood that two individuals share the same DNA sequence. Perhaps most importantly, the distribution of DNA sequences is not uniformly and randomly distributed around the globe, so my own DNA sequence bears much more similarity to my relatives and neighbours, than it will to someone arbitrarily picked from the outer reaches of rural china. These very connections, in other contexts, are used to track the progress and movement of tribes and different ethnic groups around the globe. Irish Americans, could probably identify the town from which they emigrated, if they chose to corroborate their genealogy beyond memories and documents. With knowledge of this distribution, some methods of sequencing have been estimated to have discrimination of around one in a thousand, a substantial difference on the original estimate. A further change in thinking is required, it has to be remembered that a match is not guilt, so the question has to then move on to what is the probability of guilt if a match has been achieved. Due to the inter-relationship of different DNA sequences, it may be that there are a hundred or so individuals in the area of the crime that might have provided a similar match. Whilst one chance in a billion is easy to think of as beyond reasonable doubt, ninety-nine times out of a hundred, is clearly a significant doubt.
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