A failed paternity test usually exposes instances of infidelity, but for one Washington state couple, it revealed the existence of the father’s long lost twin. The twin’s DNA, it turns out, had been absorbed into the body of his brother after he died early in the mother’s pregnancy. Thirty-four years later, still inside the surviving brother’s body, it was used to produce the couple’s child.

Last year, the couple, who has kept their identity secret for privacy reasons, was thrilled at the birth of their healthy baby boy. However, their joy soon turned to confusion when they learned their son’s blood type did not match either of their own. Because the child was conceived in-vitro, the couple immediately suspected that there was a sperm mix-up at the clinic and turned to Dr. Barry Starr, a geneticist at Stanford University, for answers. “You can imagine the parents were pretty upset,”  Starr told Buzzfeed. “They thought the clinic had used the wrong sperm.”

A DNA test confirmed the couple’s suspicions — the man was not the child’s biological father. However, he was the child’s uncle. Although, the couple was confused by the results, Starr immediately realized that he was dealing with a rare genetic phenomenon known as a chimerism. While most individuals have two sets of DNA — one inherited from each parent — chimeras have extra DNA. In some cases, such as this, the extra set comes from a long lost twin that they absorbed while still in the womb. According to The Daily Beast, chimeras can also gain their extra DNA as the result of a blood transfusion, organ transplant, or between a mother and her fetus while it is still in-vitro. Now, the medical community has acknowledged the Washington couple’s story as the first case of a paternity test getting fooled by a human chimera.

Reports of chimera are rare, but experts believe the actual condition is fairly common. According to one study, about one in every eight single births started as a multiple set at conception. One twin often dies early on in the pregnancy. In some cases, the remains of this twin get absorbed into the remaining sibling during a process called vanishing twin phenomenon.

As a result of their extra set of DNA, many human chimeras can develop two-toned striped skin, two different eye colors, or even two sets of sex organs. In fact, this father’s skin is so two-toned that as a child he would say he was a burn victim to escape teasing. Not all chimeras present these outward traits, however, and having these traits does not automatically make one a chimera. Because the lost twin’s DNA is only detectable in certain organs, it can be very difficult to determine who is a true chimera. To date, there have only been about 100 cases of human chimerism reported in medical literature, and most cases were discovered by chance.

In 2003, for example, Lydia Fairchild realized that she was a chimera after a paternity test required for welfare payments revealed she was not her children’s biological mother. Another woman, Karen Keegan, found out she was a chimera after testing for a potential kidney donor revealed she was not the biological mother of her two sons.

While it’s currently difficult to spot chimerism, experts believe it will become more widely diagnosed in the coming years. In part, this will be facilitated by the increasing accessibility of DNA testing kits, such as those from 23andMe, as well as the growing use of fertility treatments, which are more likely to lead to multiple births.