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September 13, 2005

Weird: Kids' DNA Tested, Parent Informed The DNA Is Not A Match

Obviously, this be a shock to the parent (but to no one else) if the parent were the father. It happens. Not exactly a medical mystery what may have happened.

But in this case, the parent was the mother. Who had become pregnant, carried all her children to to term, and then gave birth to them naturally. No implanted embryos or in vitro fertilizatin or other such medical intervention was involved.

Karen Keegan developed kidney problems and needed a transplant. Naturally, they checked her children for a genetic match. They found something that shocked her: her children's DNA did not match hers. She was not not their natural mother.

It turns out the woman was a "chimera;" she had two separate DNA profiles. One in her blood, another in her reproductive system. Chimerism occurs in the embryo as one is devoloping -- two fraternal twins embryos grow, but then one absorbs the other, incorporating the other's DNA into different tissues and organs of its body.

Most of her body had one set of DNA, but she had a Trojan set of DNA inside of her. And it was actually her embryonic twin sister which contributed her DNA to her children.

I first heard of this rare (?) condition on a real stumper of a CSI in which the obvious culprit of a rape did not match the DNA he left in his victim. Turns out he was a chimera; the DNA the lab tested from his blood and saliva was simply different from the DNA in his semen.

When Grissom finally got wise and figured out he might have a chimera on his hands, he got a court order for a semen sample, and then he finally got that long-anticipated genetic match.

A couple of months ago a bicyclist accused of "blood doping" (filling one's veins with other's blood to gain a competitive advantage) claimed that the reason he had two different blood types in his body was that he was chimera; the other blood type in his veins was due to his absorbing his twin in utero. He wasn't believed, but it is possible.

And chimerism may not be that rare-- some believe that there are many pregnancies that begin as twins, and most of these result in one embryo absorbing the other. If the twins are fraternal, chimerism may result.

If the twins are of different sexes-- hermaphroditism may result.

And the fact that chimerism is so infrequently checked for means that there might be a lot of false-negative paternity tests, and false-netative DNA criminological tests, too.

Lydia Fairchild's children were ordered taken from her by a judge because tests consistently showed her children were not a genetic match to her. She was pregnant with a third child, and when that child was born (under court supervision), it showed that that child too was also not a match. State prosecutors stumbled across the term "chimerism" in a medical journal and tipped her lawyer. Ultimately it was proven she was a chimera -- and that her kids were in fact her own.

I mention this because Discovery Health is currently running a documentary about chimerism, called "I Am My Own Twin," about the same woman profiled in that (yes, Dave) old NPR report.

It also tells the story of the woman whose children were almost taken from her. The medical report the prosecutors stumbled across was about none other than Karen Keegan.

Did Karen Keegan ever get her kidney transplant? Yes she did. Her children were not close enough to her genetically to donate a kidney, but, oddly enough, her non-blood-kin husband was.

Funny old world.

Chimeras and The Innocence Project: I'm a supporter of Barry Schect's Innocence Project -- using DNA testing to prove convicted criminals are actually innocent -- but I hope the possibility of chimerism is being examined in cases where there's a lot of non-DNA evidence that an accused is guilty of the crime charged.

The thing is it's very hard to prove chimerism. Lydia Fairchild's chimerism was only demonstrated to the court by showing that while her children didn't match her DNA, they did match a mix of her father's and mother's DNA. The kids had the family's DNA-- just not her version of it.

So it's not as if there's a simple test for this. The secondary DNA just lurks in different parts of the body, and isn't easy to find, even if you're specifically looking for it.

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posted by Ace at 12:49 AM

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