Yeah, on balance you're probably right. I've been reading up on it here (http://www.aclu.org/privacy/genetic/14995pub20031106.html). They list all the ways DNA evidence can get screwed up. I gather the problem with mixing isn't that it degrades the DNA (DNA doesn't fall apart if you mix two sources of it together), but that it can cause human interpretation errors. The link has a story about a case like that:
Misinterpretation of DNA tests led to the false conviction of Timothy Durham in Tulsa, Oklahoma. Durham was convicted of raping an eleven-year-old girl and sentenced to 3,000 years in prison, despite having produced 11 alibi witnesses who placed him in another state at the time of the crime. The prosecution's case rested almost entirely on a DNA test, which showed that Durham's genotype matched that of the semen donor. Post-conviction DNA testing showed that Durham should have been excluded as a possible suspect, and re-analysis of the initial test showed that the misinterpretation arose from the difficulty of separating mixed samples. The lab had failed to separate completely the male and female DNA from the semen stain, and the combination of alleles from the two sources produced a genotype that could have included Durham's. Durham was released from prison in 1997 after serving 4 years in prison.
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branchandroot
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Date: 2006-05-18 12:39 am (UTC)