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The term comes from Greek mythology, which tells the story of a fire-breathing hybrid creature, the Chimera (or “Chimaera”), comprised of a goat’s head coming out of a lion’s back and a dragon’s head coming out of its tail.

The name today is generally used to refer to the fantastical and imaginary, though, as it turns out, chimeras in biological science are both astonishing and real.

Previous chimera cases have been found in mothers who retained some of the cells of their children. Per the Daily Beast, chimeras can also arise through blood transfusions or organ transplants.

The Washington couple’s case is extraordinary because the cells that the man took from his twin were germline cells — cells with the capacity to grow into eggs or sperm, TIME reports. Thus, the man’s sperm took on a mixture of his DNA and his brother’s, with his immune system recognizing both as “the self” because the absorption of cells occurred while the man was still inside the womb.

As a result, 90 per cent of the man’s sperm is comprised of his own DNA, and 10 per cent is that of his twin. The man has another child whose DNA does match his.

“What is interesting is to explore in what way this affects the person’s feeling of identity,” Columbia University stem cell scientist Dieter Egli told TIME. “These are questions that are more important as we start to use cell therapies and cell transplantations as well.”

In the U.S., over 382,000 legal paternity tests are ordered annually, usually with the most unexpected results pointing to infidelity. In this couple’s case, however, their test results yielded even more perplexing questions about the man’s relationship to his DNA, his unborn twin (who is also a part of him) and the child to whom he is biologically an uncle even though he or she was in many ways created with his sperm.