How to get away with murder

ABC, Yahoo! and others ran a story about a woman who had a liver transplant whose blood type ended up changing. I love stories like this.

Not because of the change itself. Most likely, stem cells traveled from the new liver to the patient’s bone marrow. There, the stem cells set up shop and gave her a new blood type.

What intrigues me is what these types of stories mean for solving crimes. Because changed blood type usually means changed blood DNA. In other words, her blood cells now have different DNA from the other cells in her body. This can really confound an investigation if the police aren’t careful.

Of course this was a very rare event. But bone marrow transplants aren’t. And every bone marrow transplant results in blood cells with different DNA compared to the rest of the recipient’s cells.

Imagine that someone who has had a bone marrow transplant does something wrong and leaves blood behind at the crime scene. The police do a cheek swab to gather DNA evidence and check it against the police DNA database as well as likely suspects (including our bone marrow recipient).

The police don’t catch our bone marrow recipient because his cheek DNA is different than his new blood DNA. So he is off the hook (as long as the police don’t check the blood too). But they do get a match and arrest someone—the donor.

Sounds weird but something almost like this complicated a case in Alaska a few years ago. There was a serious crime and a semen sample from the crime scene matched a known criminal’s DNA. But the person whose DNA matched the DNA from the crime scene had a strong alibi…he was in jail at the time! So what happened?

A little further investigation showed that the guy in jail had received a bone marrow transplant from his brother. And his brother was the one who committed the crime.

This one worked out all right in the end. But what would have happened to the brother if he weren’t in jail at the time? Would an overworked public defender have figured something like this out? The guy was lucky he was already in jail!

So people with bone marrow transplants need to be careful. And the police need to be careful about what sample they take from suspects.

Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.

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