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[Corrected Sept. 20, 2013 per comment from Mr. Zimmer. In most cases, the two types of DNA are not in equal proportions in the body. Hence, there is not a 50-50 chance of drawing the “right” DNA.]
A recent New York Times article by the esteemed science writer Carl Zimmer (“DNA Double Take” Science section, p. D1, September 17, 2013) brought some interesting new developments in DNA science to my attention. Scientists have now determined that some women are “chimeras” meaning they host two different DNA types. In one case, a woman with three children underwent DNA tests that seemed to show only one of the kids was hers. Further tests on additional samples from her showed she was, in fact, a chimera and that all three children were, in fact, hers. (I’m pretty sure she knew that already, but I suppose it’s nice to have it confirmed.)
But it’s even more complicated. Apparently bearing children leaves some elements of their DNA behind in mom’s body. Y-chromosomes have been found in some women’s breasts. Others have had a child’s DNA appear in tests.
The examples in Zimmer’s article all appear to be women. However, based on his descriptions of the various ways a chimera can be created, my guess is that there are at least a few men with multiple DNA types. (Mr. Zimmer was kind enough to confirm my speculation in an e-mail response.) This is a story about DNA fluctuations and their implications — or, at least, possible implications.
This new evidence leads to all sorts of speculative conclusions. Please consider the rest of this article science fiction.
Scientists have known for years that there is a negative correlation between having at least one child and the probability of developing breast cancer. This new evidence gives rise to a question: is there a difference between having a son and having a daughter?
Only men carry the infamous Y-chromosome. This chromosome has been found in the breast tissue of some women who have at least one son. A speculative guess is that the Y-chromosome provides some protection against breast cancer.
I went looking in the public data. There are mountains of numbers available. But I found only one questionnaire that would even let me infer whether a woman had borne a son. (There were two questions: number of children and number of daughters. Clearly the difference should be the number of sons.)
A simple chi-squared test should be enough to determine whether my hypothesis is worth pursuing. I could not find the data, but I know for sure that there are many people out there who know much more about this subject than me.
Congratulations, female criminals. You have a slightly better chance of getting away with it if DNA evidence is used. If you are a chimera, there is a chance they will pull the wrong DNA. Male chimeras are probably not as common, so sorry guys — you’re still on the hook.
This could, in principle, be solved by taking samples from more than one location. But the percentage of DNA types varies and is not 50-50, meaning many samples may be required. And, according to Zimmer, hair seems remarkably reliable. Every strand of your hair probably has the same DNA. So don’t leave any hair behind at the crime scene.
Maybe you weren’t adopted after all. If either parent was a chimera, DNA matching gets tricky. So if you think, “They’re not my real parents” and your suspicions seem to be confirmed by DNA, you may find your parents demanding a second, or even a third, test.
Birth Order May Matter
By definition, the first child cannot acquire any DNA from a previous child. However, subsequent children are vulnerable to contamination. In fact, my guess is that child number 3 is more likely to be contaminated than number 2 because there were two previous kids making DNA donations to mom. (I am the oldest child and my wife is an only child, so we can speculate on this without fear. My younger brother, on the other hand, … well, I’ll be sure to let him know.)
This is off-the-top-of-my-head speculation. If my comments here trigger some actual research, I will be thrilled. Thanks again to Carl Zimmer for his lucid explanation of a complex topic.