With all due respect, your argument carries a definitive prosecution bias. You make no room for the possibility that certain portions of DNA evidence may have been innocently collected improperly (as an example the DNA from the bra clasp) or innocently analyzed improperly ( as an example the Low quantity DNA from the knife where only one sample was run and no controls are available for the defense or the court to even look at) while other DNA evidence was collected and analyzed properly (ie- AK's low quantity DNA on the knife and much of RG's DNA in the murder room. In this case it just so happens that the knife DNA and the bra clasp DNA are critical pieces of evidence. Your argument also doesn't take into account the POSSIBILITY (please don't jump on me too hard for this) that there might be a prosecution bias from the people doing the collection, anlysis and storage that would permit "mistakes" to be made with critical pieces of evidence. In other words the forensic scientists MIGHT have an a priori expectations of what the results of their testing should reveal. Any scientist worth his salt will tell you that such apriori expectations can lead to fallacious results if extreme attention to detail in performing tests and analyzing the data is not paid.
I would argue that a jury made up of people like yourself (maintaining the arguments that you made above) would hamstring a defense team. They could NEVER or almost NEVER win an argument with you questioning prosecution evidence even when that evidence is scientifically suspect.
bbm
Yes, I believe most people would question why certain samples are vulnerable to contamination whereas others collected and tested in the same way are not. Example: Meredith's DNA on knife, and Amanda's DNA on knife.