this is the first installment of a science based weekly post intended to complement Brendan's weekend Art threads. While the topics are not intended to be strictly partisan, I hope to choose topics that have broader implications beyond mere knowledge of the physical action of the universe. Feel free to delve into the politics in comments. :)

Human Chimera

There are a number of metrics which are associated with the concept of identity. Your birthday (and consequently your age), dental records, fingerprints, retinal patterns, and so on, are considered part of a biometric signature that helps distinguish you from everyone else. Indeed they can be said to be the source of your identity, from a legal standpoint.

The king of biometric data is your genetic code, as expressed by your DNA and RNA. Everyone has DNA, whereas not everyone has fingers, for fingerprints, or eyes for retinal patterns. Age is not easily testable in case of dispute, whereas DNA testing has become routine. Similarly, dental work can be faked, whereas the ability to reprogram our DNA through retroviruses is still immature.

DNA is used in the judicial system now to establish guilt or innocence, in some cases releasing wrongly convicted people decades after their mistaken incarceration.

"Juries think science is so absolute that DNA evidence is without question," says Charles Leonard, a partner at Tremper, Bechert & Leonard in Fort Wayne, Ind., and a court-appointed defender in Indiana v. Hopkins, the third case involving DNA evidence ever tried in the country.

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DNA testing would seem to be a god send; nearly infallible (coincidental matches for properly done DNA tests vary from 1 in 100 thousand to 1 in 1 million, easily good enough to select from a limited pool of suspects), universally applicable, fairly fast, and reasonably priced. Unfortunately it's based on a false supposition.

You aren’t just you.

EXPLAIN this. You are a doctor and one of your patients, a 52-year- old woman, comes to see you, very upset. Tests have revealed something unbelievable about two of her three grown-up sons. Although she conceived them naturally with her husband, who is definitely their father, the tests say she isn't their biological mother. Somehow she has given birth to somebody else's children.

This isn't a trick question - it's a genuine case that Margot Kruskall, a doctor at the Beth Israel Deaconess Medical Center in Boston, Massachusetts, was faced with five years ago. The patient, who we will call Jane, needed a kidney transplant, and so her family underwent blood tests to see if any of them would make a suitable donor. When the results came back, Jane was hoping for good news.

Instead she received a hammer blow. The letter told her outright that two of her three sons could not be hers.

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So what happened? Turns out that "Jane" had at least two distinct genetic codes in her body. She was made up of two people, if we accept that DNA determines identity. This was not a function of a transplant or transfusion. Jane had a non-identical twin in the womb. At some point her body absorbed her sister and the two became a Chimera.

Jane, and her parents, most likely never even knew about her sister that would leave this legacy. Chimeras are well known to biologists. A simple example is to take a cutting from one plant and graft it onto another so that they become one individual plant but with two distinct genetic codes. Modern genetic engineering focuses a great deal upon the creation of chimera. As far back as 1984 a chimera was created by mixing sheep and goat into the unfortunately named Geep.

Despite being well known, it was thought until recently that Human Chimeras were rare. Recent work suggests this is not the case after all. Some estimates now say as much as 8% of people who have non-identical twins will have some degree of chimerism due to exchange of cellular material in the womb. That number may very well rise, and it does not include people like Jane, who didn’t know there was a twin at all, or people who absorb foreign cells through transplants or transfusions. The problem with detecting chimerism is that it could be any subset of your body that has the different code or codes. In Jane's case apparently the marrow cells that produced the blood she gave had a different code than those of her ovaries.

From a legal standpoint the good news is that chimerism is going to produce false negatives (as with Jane and her kids that "couldn’t" be hers) rather than false positives. Perhaps more interesting though is what this means in a broader context. If biometrics are more and more often becoming integral to our understanding of identity, what does it mean that the holy grail of biological data is so compromised?

This is not academic. With a future of direct human cloning in our immediate future the question of identity becomes quite interesting. With all due respect to Descarte, western society has mostly discarded the concept of identity being entwined with memory or conscious activity (a person in a coma remains a person and is not an object, even if said coma is permanent). If we discard attributes associated with conscious thought the next most likely candidate would be where we are now- biometrics, or attributes associated with the physical body. If these are similarly incapable, or undesirable, where do we go next?

Or to put it another way: is Jane herself, or her sister, or both, or neither?