Imagine that one of your parents was someone who never existed. Sound like science fiction? It's not. Because of a rare genetic blending effect known as a "chimera," it is possible for genetic material from a twin lost in utero to be absorbed into the surviving twin. If those cells become sperm or eggs, then it's possible that those sperm or eggs could go on to create a child whose parent is a twin that was never born. In other words, it's possible that a person's son or daughter could actually be his or her niece or nephew, as was the case for this Washington couple.
This is complicated news for people taking paternity tests. For people who are unknowingly chimeras, the news that they are not the biological parent of their own children could be liberating or devastating, even if only partially true.
While the idea of being a chimera might be intriguing, chimeras can face real problems when genetics conflict with law. For example, a woman in Washington discovered that she was a chimera after genetic tests required for public assistance revealed that she was not technically the mother of her children. She was accused of fraud and threatened with the loss of her children before additional genetic tests revealed the truth.
Men undergoing paternity tests to gain rights of visitation and custody of their children could potentially face challenges if they are unaware of being a chimera, and their paternity tests come back negative. In other words, it's technically possible to lose rights pertaining to your own children because of a genetic test that examined the "wrong" cells. A child could similarly lose rights of inheritance and child support if a paternity test revealed his father to be his biological uncle.
This is not to say that traditional paternity tests are useless; rather, in some cases, they simply may not provide the whole story.