In our last two posts – Part 1 and Part 2 – we looked at what DNA is, where it is found, and how it is tested. If you have not yet read those two posts, I encourage you to do so before checking out this final post. Part 3 brings us back around to our initial question: “How does DNA evidence exonerate the wrongfully convicted?” The answer to this question is much more case-specific and requires a closer look at where the DNA was found and how it is connected to the crime.
As discussed in Part 2, DNA can be found in many places at a crime scene, such as saliva on a cigarette butt or drinking glass, drops of blood on the floor, or sweat on the collar of a shirt. But not all of this DNA is helpful in determining who committed the crime. Maybe the saliva on the cigarette butt and the drops of blood were from the victim, and maybe the shirt was also the victim’s. This does not tell us anything about who the perpetrator might be. In order to rule out this possibility, a sample of the victim’s DNA is always included in testing.
On the other hand, what if the crime alleged is rape and seminal fluid is found either on the rape kit or on the victim’s bed? If analysts are able to get a DNA profile from a sample of that fluid, this could potentially tell us exactly who the perpetrator was, or, in the case of exonerations, who it was not. Of course, it is not always that simple. Other considerations, such as whether the victim recently had consensual sex with someone, can muddy the waters.
Likewise, attempting to exonerate someone using DNA found on a handheld object, oftentimes a murder weapon, presents a host of complications. Remember in Part 2 when we mentioned “touch” DNA? Sometimes when we grab or touch objects sweat and oil from our hands can transfer to the object – just think about fingerprint smudges that end up all over a glass door from pushing it open and closed. Our DNA can be found in those smudges. The problem is, we probably were not the only ones to open and close that door. Our DNA is mixed in with every other person who pushed that door open. The same can be true for a kitchen knife that becomes a murder weapon. If a swab of the knife’s handles contains DNA from multiple people, it can be hard for analysts to distinguish between the different profiles. Even if they can, it can then be hard to say who held the knife to merely cut up a steak and who held it to stab the victim.
Assuming the victim has been ruled out as a source, no mixture of multiple people’s DNA is involved, and we know for certain that the unknown sample could only have come from the perpetrator, it comes down to a comparison between DNA profiles – the unknown sample from the crime scene and the convicted person’s sample. If the unknown sample’s DNA profile is not the same as the convicted person’s DNA profile, they got the wrong person.
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