txsvicki said:
I can't seem to remember all the things that have been said about dna. Was dna under JonBenet's fingernails the same as that found on the panties?
Say this after me: Overlap is not matching.
One marker under a nail from one hand was found that had the same alleles as an equivalent marker from the underwear. Two markers from JonBenet's other hand were found to have the same alleles as two equivalent markers from the underwear. (In other words, the letter A was found on JonBenet's left hand, the letters A and B were found on her right hand, and the letters A, B, and C were found in the underwear.)
Now, let us use this example. You have red hair. I have red hair. At the crime scene, DNA is found under the victim's left hand nails, and that DNA belongs to someone with red hair. Does that mean you and I are both the same person
and the person who committed the crime, because our hair is red like that found in the DNA at the crime scene? You and I would both say no, because we know we did not do it. The DNA is then examined and found to have markers from a sample under the victim's right hand belonging to someone with red hair and freckles. You have red hair and freckles, but I have red hair and no freckles. This establishes that if our DNA was compared to the crime scene DNA from the right hand, I am exempt, but you are not yet. Then, another DNA sample from the victim's underwear is examined, and it reveals the killer had red hair, freckles, and violet eyes. You have red hair and freckles, but green eyes. You are now eliminated. But there are diehard theorists out there who will insist that you and I are the same person, and that we (as this theoretical single person) killed the victim, because the killer had red hair and we have red hair.
See? This is what intruder theorists hang part of their theory on. The just-one marker from one of JonBenet's hands that could be found
overlapped the just-two markers from JonBenet's other hand. But like your red hair and mine overlap as part of our visual appearance, it does not mean that our red hair belongs to just one person. This is why criminal investigation requires 13 markers, people. Many people have individual markers in common with any DNA sample, but it becomes very unlikely that
all 13 markers will overlap and therefore count as a match.
And, of course, the whole ballgame gets complicated when there is not even any certainty that the markers found belong to just one person. Introduce contamination and innocent transfer to the mix, and what you have is the famous "not a DNA case."