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March 23, 2011 6:59 PM   Subscribe

How common is human chimerism of the sort described in this Boing Boing post? What implications does the existence of chimerism have for DNA testing, especially with respect to the criminal justice system? Does it pose a practical limit on the usefulness of DNA evidence, or just a theoretical one? Has chimerism come up in any cases, or been considered as a problem in legal scholarship?
posted by gerryblog to Science & Nature (14 answers total) 8 users marked this as a favorite
I don't think anyone knows how common it is. There isn't really any kind of statistically-valid sample from which to judge.
posted by Chocolate Pickle at 7:05 PM on March 23, 2011

Response by poster: Maybe the followup question is: Why not? It seems like the sort of thing you'd want to lock down about DNA before you start drawing larger conclusions.
posted by gerryblog at 7:13 PM on March 23, 2011

the wikipedia post you link discusses a few cases in which it has had legal significance. as for 'why not?', i'd say because it's probably rare, and thus there's not a good systematic way to assay for it, short of randomly testing huge segments of the population over large parts of their DNA sequences. difficult to get approval for, that.

maybe someone with more genetics experience than i could think of a better way to test for this?
posted by Tandem Affinity at 7:17 PM on March 23, 2011

Why not?

One problem is that even in people who are chimeras, it isn't always easy to tell where part A is and where part B is.

It isn't always 50:50. It certainly isn't necessarily left-side/right-side. It can be top-half/bottom-half. It can be inside/outside. It can be one-small-piece/all-the-rest.

So if you have no clinically-obvious dimorphism, then just how many samples, and from where, do you take from each subject in order to determine if there are two genomes present, or only one?
posted by Chocolate Pickle at 7:28 PM on March 23, 2011 [4 favorites]

My SO the biologist says human chimerism is surprisingly common, like a few percent of the population.
posted by hattifattener at 7:31 PM on March 23, 2011

IANAL or geneticist, but I would imagine that DNA from a chimeric individual would still be identified as a sibling. (In other words, DNA collected from a crime scene tested against the DNA from a suspect would show that the two are related.) That seems to me that it would still lead to a fairly straightforward investigation of known siblings, parents, half-siblings, etc, and then if none of those individuals are a match, and there is no evidence of a missing/surrendered sibling, a medical exam.

But that's only if the DNA doesn't match. If it does match, the only circumstance I could imagine is if you had a fraternal twin who absorbed your identical twin in utero (i.e. you were actually triplets--two identical and one fraternal). I suppose the existence of chimerism could, in that case, be sufficient for reasonable doubt (although if chimerism were introduced into the trial, a savvy prosecutor would probably be able to get a court order to get the twin examined for chimerism).

In other words, it strikes me that the law can probably cope with this already, the investigation will just take longer.
posted by thinkingwoman at 7:46 PM on March 23, 2011

As genetics has moved from the chromosomal to the molecular levels of analysis, chimerism/mosaicism/de novo mutations are more common than previously suspected. The problem with testing for mosaicism though is that you are testing for mosaicsm: if you do not find it, does that mean the person is not mosaic or that mosiaicism was not present at a detectable level in the specimen tested by the methodology used? Both for the suspect and for the other family member. Which goes back to Chocolate Pickle's comment about when and where do you test/stop testing?
posted by beaning at 7:55 PM on March 23, 2011

In short, it sounds like the other cells can be anywhere, so when do you stop looking And mothers can absorb some of the cells from their offspring, increasing the chance for some level of chimeraism.
posted by filthy light thief at 8:40 PM on March 23, 2011

Response by poster: Given everything that's being said, I'm surprised the default assumption isn't that *everyone's* a chimera, provided you look hard enough.
posted by gerryblog at 8:52 PM on March 23, 2011 [1 favorite]

There was a TV special on the real-life implications of chimerism-- including a woman who had to fight for custody of her children because in a DNA test for public assistance, she didn't match her kids.
posted by elpea at 9:02 PM on March 23, 2011

Best answer: Tyler Hamilton's defense team claimed chimerism, and not illicit blood transfusion, was why he failed a doping screen. He was suspended and his appeal was dismissed.

Quote from the linked article:
Dr. Ann Reed, chairwoman of rheumatology research at the Mayo Clinic, who uses sensitive DNA tests to look for chimerism, finds that about 50 to 70 percent of healthy people are chimeras. The more scientists look for chimerism, the more they find it.
posted by momus_window at 9:59 PM on March 23, 2011 [1 favorite]

Best answer: As far as I know this is still the seminal paper on the subject,
Charles E. Boklage's Embryogenesis of Chimeras, Twins and Anterior Midline Asymmetries

Dr. Ann Reed appears to have yet to publish anything to back up her claim, but Boklage pretty justifiably, says that it is at least 10%.

For a good discussion of legal questions I would look here:

Also of interest might be:

Children can have maternal cells into their forties, page 5 (PDF)
C. Spencer et al., Report From the Ross Petty Pediatric Rheumatology Symposium: Old
Challenges & New Directions in Pediatric Rheumatology, Apr. 2, 2005, 195, 199.

Fetal cells have been shown to persist in the material circulation for up to 27 years after pregnancy
J. Lee Nelson et al., Michrochimerism and HLA-Compatible Relationships of Pregnancy in Scleroderma

Mothers can carry the cells of their sons in their livers
Anne Stevens et al., Liver Biopsies from Human Females Contain Male Hepatocytes in the Absence of Transplantation

If you lack library access and want copies of any of these papers, just memail me an email address and I can send you a PDF, we are discussing them academically right?
posted by Blasdelb at 8:54 AM on March 24, 2011 [1 favorite]

As an aside, I remember an anthropology professor discussing intentional chimerism as a theoretical possibility for the offspring of gay couples, in the context of the future of genetic engineering. Two eggs would be fertilized by each of the fathers, then the zygotes combined prior to implantation in a surrogate so that the resulting offspring would be genetically related to both fathers. I don't know if the combining of zygotes part is scientifically possible yet, though.
posted by Safiya at 11:01 AM on March 24, 2011

There's a Radiolab episode about a woman who according to a DNA test wasn't the mother of her children: So-called Life.
posted by amf at 11:33 AM on March 24, 2011

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