You may remember learning in biology class that the combination of genetic material from both of your parents is part of what makes you unique. The combination of DNA from your parents becomes your personal genetic code that you will pass on if you have children. But in some rare cases, a person can have both their own DNA and a second set of DNA that is not theirs. Sometimes this foreign DNA did not come from their parents. This is known as being a chimera.
The word “chimera” comes from Greek mythology, where a chimera is a fire-breathing, dragon-like creature with a lion’s head, a goat’s body and a snake’s tail. But, aside from their genetic makeup, people who have chimerism don’t look different from other people. Most people don’t even know they have more than one set of DNA and only find out through medical testing for unrelated reasons.
One of the primary causes that has been identified for chimerism is being part of a twin pregnancy. Very early in the pregnancy—sometimes the parents don’t even know they are expecting twins—one of the fetuses doesn’t develop and absorbs back into the womb. In this case, some of the DNA from the undeveloped twin merges with the genetic material from the developing baby. One woman in California found out she was a chimera when she had genetic testing to learn more about the large birthmark on her torso. The difference in skin color she always thought was a birthmark came from her twin’s DNA and was evidence that she is a chimera.
Scientists recently learned that you don’t necessarily need to be a twin to be a chimera and that in extraordinary circumstances chimerism may develop later in life. This gave a Nevada man the surprise of his life after he had a bone marrow transplant to treat leukemia. Years after the man went into remission, routine testing revealed that except for the DNA in his chest hair and the hair on his head, all of his other DNA came from his bone marrow donor. Through the lifesaving treatment, the donor’s DNA replaced his own and the man had become a chimera.
In addition to the physiological questions that come up from the concept of chimerism, the discovery that organ donor DNA may “take over” the DNA of the recipient can have intriguing consequences for the legal system. The Nevada man who became a chimera worked in a law enforcement office, which opened his colleagues’ eyes to the fact that DNA evidence to prove or disprove a crime may not be accurate if an alleged criminal is a chimera. Forensic scientists are already exploring this unique dilemma.
For everyone else, knowing that chimeras are not just mythological creatures puts another spin on the phrase “Just be yourself!”