The Mind-Blowing World of Human Chimeras

Maggie Koerth-Baker is a guest blogger on Boing Boing. A freelance science and health journalist, Maggie lives in Minneapolis, brain dumps on Twitter, and writes quite often for mental_floss magazine.

One person outside: But two people "inside": That's the gist of the chimera, a human being who carries the DNA (and sometimes the body parts) for two. It sounds crazy, but it happens. In fact, doctors think it probably happens more often than we realize. Unless there were some reason to test the DNA from cells in different parts of your body, you could easily be a chimera and never know it. Happy Freaky Friday, everybody.

So how's it happen? In this excerpt from my book, Be Amazing, I explained how chimeras happen, and how confusing it can be to be one.

First: Get That Meddling Sibling Out of Your Way

Imagine you're a fertilized egg, just a few days old. There you are, floating around the womb and minding your own business, when, BAM! You run smack into another just like you. Well, not just like you. But certainly close enough to be a threat. Now, you have a choice. You can roll over and let yourself be born as just another fraternal twin, or you can stand up for your individuality and absorb the interloper. Naturally, you do the smart thing, and nine months later your parents take home one healthy baby.

Then: Discover That They Aren't As Dead As You Thought

Like a horror-movie villain locked into a three-picture contract, your twin never really died. Instead, she'll end up hiding in plain sight–within your very cells–rendering you a chimera, a single human who carries the genetic makeup of two different people. Most of the time, there aren't any outward signs that your body is harboring a stowaway. But when you do notice, things get a little crazy. Take Karen Keegan, who discovered her chimera-ness at age 52. When Keegan needed a kidney transplant, she and her two adult children underwent DNA testing to figure out which kid's kidney would be the best match for mom. Surprisingly, the tests showed neither. In fact, according to DNA, Keegan's children weren't her children at all. The case confounded doctors for more than two years until, in 2000, the docs finally realized that Keegan's blood cells carried different genes from the cells in her ovaries—the long-absorbed twin was found.

Perhaps you're wondering whether chimeras can incorporate twins of two different sexes. The answer is yes, and the results are often much stranger. In 1998, Scottish doctors reported treating a teenage boy for an undescended testicle. But when they put the kid under the knife, no second testicle could be found to pull down. Instead, where the ball should have been, doctors discovered an ovary and fallopian tube. Chimera strikes again.

For some fun further reading, check out the story of Lydia Fairchild. Like Karen Keegan, Fairchild's chimeric nature was discovered after DNA tests said she wasn't the mother of the children she was pretty sure she remembered giving birth to. Unlike Keegan, however, Fairchild's kids were still young and the initial DNA test almost cost her custody.

Much like Professor Xavier of the X-Men, illustrator Michael Rogalski is locked in deadly, psychic battle with his evil, chimeric twin.