DNA is in every cell of your body and it's basically the "blueprint" for your entire body. That's right. Every cell holds a full blueprint of every part of your body. That's why DNA is unique. Just look at you. You're unique. So is everyone else (except twins, trips, etc.) and that's why each person's DNA can be a clue as to who did a specific crime if a specific DNA is found at or in the scene of a crime.
The basic DNA test does not try to include people. Its intended to exclude people because if a particular allele is missing from the tested cell and you cell has that allele, then you are excluded. Or the reverse can also hold true.
Generally, only about 4-6 alleles are tested because they comprise the best and most significant results. If there is uncertainty in the outcome, they can test more alleles to be more specific.
When DNA excludes a person from a crime it's full proof
When a part of the DNA found in the victim is missing from the alleged perpetrator of the crime, that simply, purely and cleanly excludes them from being the depositor of that DNA sample (it could be blood, semen, skin, hair or almost anything from one's body). When that happens, it means that person was not the depositor and therefore, the logical conclusion is they had no part in the crime.
This was the original intention of DNA testing. To exclude non-criminals from being wrongly convicted of a crime that they didn't do.
When prosecutors INCLUDE a person using DNA analysis
Prosecutors first learned how to exclude people from crimes using DNA, but that wasn't the only purpose it could be used for by them. They started INCLUDING Defendants in unsolved crimes by getting the Legislatures to pass laws that now require all people convicted of Felonies to give a DNA sample so they can run them through a computer and cross check for DNA that came from those unsolved crimes. If they get a "hit" they will then start an investigation to see if they can prove a case against the depositor of the DNA containing bodily sample. If that person wasn't in jail or in front of a huge number of people lecturing, they can easily become a newly discovered Defendant and be charged with the new crime.
When prosecutors misuse DNA to prove their case
Some Prosecutors have learned how to misuse the statistical analysis of DNA test results. If they really want a particular subject whose DNA is similar, but not exactly perfectly a match, they will sometimes get the scientific examiner to offer a statistical analysis that incorrectly concludes that the man before the jury "could" be the right person who left that DNA sample behind at the scene of the crime.
They do this by misrepresenting the DNA analysis and misleading the judge and jury with statistics that really don't prove what they're saying. While this article doesn't allow for a full explanation of this (since it is quite scientific as well as statistical in nature) it IS subject to successful cross-examination that will completely undermine the incorrect conclusions drawn.
Always get your attorney to hire a DNA expert to testify in this type of case. It helps explain the matter to the jury so they understand it and won't get misled.
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