November 23, 2005

DNA testing is dead, long live DNA. DNA is infallibale, right? Not when we have Hybrid Humans (NPR audio) also know as Natural Chimerism.

To make a complicated story short, some people have more than one set of DNA within them. To quote the first link:

Because these Chimeras have TWO unique types of DNA in their bodies, they can potentially, for example, have a kidney that is one type of DNA, ovaries that contain another type of DNA, and their blood may have a different type than their ovaries and so on.
No one has any idea how common of occurance this is. Even you could have more than one set of DNA floating about in your body. Just like finger prints, nothing is absolute, and the reality is once again stranger than we could imagine.
  • Um... having two sets of fingerprints doesn't mean you can't be identified by either or both of them.
  • Not double post, but see also.
  • I saw that CSI. It was kinda weird. But they got the guy in the end.
  • Oh, yeah and in that National Geographic article (2nd link) it says, "Just because you're doing DNA testing, people shouldn't always consider the results 100 percent reliable" So they aren't saying it's infallable. It does eliminate millions of potential suspects though ;)
  • See also the links I put here for DNA testing fallibility and legal usage. The last link doesn't apply to this topic. And this doesn't invalidate DNA testing because DNA evidence is only one piece of evidence used. If the results are inconclusive then it doesn't make good evidence, this happens without chimeras. Hopefully a stronger case can be made by not relying on one piece of evidence.
  • Apologies for double post, but this struck me as absolutely hillarious and I forgot to add: "How does Chimerism come into play regarding the topic of homosexuality?" And the other thing that struck me: Couldn't someone use mitochondrial DNA to eliminate certain suspects? I really don't know what kind of diversity there is in human beings regarding mtDNA; anyone know?
  • You could definitely use mtDNA to eliminate suspects, but not to confirm, as mtDNA is inherited directly from your maternal line - any one who has the same maternal lineage as you will have the same, or virtually the same, mtDNA. As far as DNA testing goes it is far from dead. A positive match of a human genetic fingerprint is still a positive match with the chance that another person has an identical genetic fingerprint being 1 in 82 billion (except in the case of identical twins). Negative results however, are slightly less conclusive, but only slightly as chimeric individuals are extremely rare.
  • I was thinking about DNA evidence yesterday morning. A local Senator was on WPR talking about legislation to reintroduce the death penalty in WI under certain circumstances. The bill includes language requiring "DNA evidence" to be present. What I realized is, DNA evidence is only as good as its documented provenance. An analogy could be taken from carbon dating in archaeology: you can say with some certainty that a piece of wood is 2800 +-30 years old, but depending on how disturbed the site is, that information may contribute little to determining how old the site actually is. Similarly, a hair could be identified as belonging to John Doe, but if you don't know when and how it got where you found it, it doesn't give you any of that information about John Doe.
  • There is a lot of attention and concern about DNA evidence because it may be slightly less than 100 percent accurate. But no one seems to care much about the much more common and far less accurate eyewitness testimony. Look at those statistics. Thirty percent of violent crime victims can properly identify the perpetrator. Thirty percent. And eyewitness testimony is the strongest piece of evidence in thousands and thousands of felony convictions. Where is the outrage over this? It is much more serious than the DNA issue. Also note the very interesting fact that the confidence level of the eyewitness is not a strong indicator of accuracy.
  • I'm famous!
  • You could definitely use mtDNA to eliminate suspects, but not to confirm, as mtDNA is inherited directly from your maternal line - any one who has the same maternal lineage as you will have the same, or virtually the same, mtDNA. Yes, but we have used mtDNA to source Mitochondrial Eve 150K years ago. My understanding is that there is much less variation in mtDNA due to asexual propagation. I was wondering about the extent of mutation in mtDNA among the current world population. In DNA you get a 50/50 contribution over a generation because of the combined parental contribution of genetic material. Is there enough variability by mutation in the short code that mtDNA is to eliminate suspects reliably? I would cite this but it seems misinformative by not explaining that the mother's mtDNA is also passed to sons (and no further), not just daughters -- unlike the only patrilinear or hermaphroditic occurance of Y-chromosomes for testing of that nature, of course. Regardless, evidence is about eliminating doubt, not confirming truth. It's just the way most justice systems work. DNA testing itself is just a statistical measure based on matching enough sets of codons from samples to eliminate doubt, not confirm truth. Just a thought: If sperm cells are at the scene of the crime, test sperm cells of the subjects. If it's blood, test blood. Shouldn't that eradicate chimeric DNA from multiple body organs?
  • Subjects, suspects, your majesty.
  • It explains Ricky Martin
  • Turkey!
  • I saw a documentary where they used mDNA to test whether the Anna Anderson, who claimed to be Grand Duchess Anastasia, was for real. They compared her mDNA to that of the Duke of Edinburgh who were related matrilineally (sp?). It turned out that Anderson was not Anastasia but Franziska Schanzkowska. Of course, her supporters dispute the claim and say that the DNA sample was tainted. I couldn't find any mention of the documentary, but here's the wikipedia entry.
  • Interesting link, meredithea, thanks! Still hazy on the human variability in mtDNA.
  • Just to put you all at ease; I did it. So if anybody comes snooping around your place looking for DNA, just send them to my house. I got lots, and I'm guilty as hell.
  • Did you do it to get into the national databank, mj? Do an inside job?
  • MCroft-come-lately here, but reliability of DNA evidence can be degraded significantly by poor handling and test procedures by the police and the labs. Houston, Texas, had their lab de-certified becuase underfunding had led to poorly trained technicians and tainted samples. Not that our local prosecutors told juries anything other than the "100% reliable" line.
  • Yeah, I kind of linked to this disclaimer, MC (see the links where "this kind of stuff happens"). Do you have a link for the Houston lab?
  • AFAIK, InsolentChimp, and I may not be answering your question at all, mDNA is inherited directly from the mother to child. There's not the genetic potluck that occurs when the mom's and the dad's DNA mixes up. The only changes in mDNA occur when mutation occurs, so I would assume that the rate of variability in mDNA and the rate of general mutation in humans would be roughly the same. I just Googled to check my idea, and Ask A Scietist seems to agree with me.
  • Except I can't spell. Scientist.
  • Here's a link that discusses the possible reopening of the Houston crime lab. I remember when it was closed in 2002. The trustworthiness of DNA evidence was never disputed in this case. Instead, the lab had several under-(or badly-)trained technicians who were doing tests and storing evidence incorrectly. I guess the question becomes one of if the evidence is too fragile to make perfect (or optimal) test conditions possible. Does one have to spend tons and tons of money to achieve the optimal conditions of staff and lab, or is it obtainable under a run-of-the-mill city budget? I may not be making sense. Can you have tryptophan hangover?
  • The only changes in mDNA occur when mutation occurs, so I would assume that the rate of variability in mDNA and the rate of general mutation in humans would be roughly the same. This is what I was getting at, rate of mutation in mtDNA. No offence, meredithea, but I wrote everything you just said earlier. A prof told me this week that mtDNA mutates at a much higher rate than normal DNA, but she's getting on in years and sometimes forgets to mention things or confuses them with others. All there is on that ask a scientist link to answer my q.: "I don't know what the mutation rate of mitochodrial DNA is, but I would "guesstimate" that about one in 1,000 individuals would carry a newly arising mutation when compared compared to their mother." As much speculation as possible. Still fuzzy, I'll keep searching.