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Halides,
To be fair to Barbie, I don't think Barbie is here arguing that tissue type can be determined by peak heights. She seems to be arguing that the quantity of Amanda's DNA is greater than the quantity of Meredith's DNA in some of the blood samples. Amanda's traces are "higher" than Meredith's in some of those spots of blood, so the blood is "mixed." The peak height is, apparently, a function of DNA concentration in the sample.
This seems like a silly argument when applied to diluted blood.
And the pink "mixed" blood appeared to be diluted blood. And if diluted blood, diluted blood DNA, too. (Because fewer white blood cells.) So any non-blood DNA added to the diluted blood drop would have made a larger contribution than if the drop had been undiluted blood.
Let's suppose it was Meredith's diluted blood. If there were more residual non-blood DNA in the bathroom left by Amanda than left by Meredith, a "mixed" sample could have had "higher" Amanda traces without any of Amanda's blood occurring in the sample. Maybe Amanda "shed" more DNA than Meredith did. (As a hippy chick, did she spit more?) Oh,...or maybe Amanda had lived in the cottage longer than Meredith. So many ways to create diluted "mixed blood" without the bloods of two people mixing.
To conclude that a sample was mixed blood, one would have to know the background distribution of Amanda's non-blood DNA and Meredith's non-blood DNA where the sample was collected. That was never discovered. Therefore mixed blood wasn't discovered either. Only mixed DNA traces were discovered. Exactly what one would expect.
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