A baby boy born in Washington last year is his own cousin, geneticists found. The child’s father is a genetic chimera—a person who carries two sets of DNA. After failing several paternity tests, the man learned that “his” sperm actually belonged to a twin he had absorbed in the womb. The vanished twin, therefore, was technically the baby’s father [PDF].

Most people with chimerism will never find out. The process of absorption takes place so early in fetal development that there are rarely outward signs. Two eggs become one, and normally no one is the wiser. Sometimes a chimeric person will have two differently colored eyes. In cases when one egg is male and one is female, the resulting baby may be intersex, but even that presents in varying degrees of obviousness. The skin of some people with chimerism may even be striped or swirled where the two lines of DNA met, but again, this is rare.

This isn’t the first case of its kind, although it is the first to focus on a father. Karen Keegan and Lydia Fairchild were the subjects of intense scrutiny after DNA tests showed that they couldn’t possibly be the mothers of their own children. Fairchild was accused of kidnapping and nearly lost her case. Only when doctors read about Keegan’s case did they consider the possibility that Fairchild might be a chimera. Genetic testing bore out the theory, and Fairchild got to keep her kids.

The parents in the most recent case first suspected something was up when their son was born, reports BuzzFeed's Dan Vergano. He was healthy, but his blood type didn’t match his father’s or his mother’s. A cheek-swab paternity test confirmed that the child’s DNA did not match his father’s. The couple, who had conceived through in vitro fertilization, worried that their fertility clinic had used a stranger’s sperm by accident.

They checked with the clinic. Staff assured them that there had been no mix-up. So whose sperm was it, exactly? The couple took their question to Stanford University geneticist Barry Starr, who runs the Ask a Geneticist website. Starr suggested a more comprehensive type of genetic testing. Those results showed that the baby was definitely related to his father—both as a nephew and a son.

But the man had no brother … not that he knew of, anyway. He did, however, have striped skin.

More tests confirmed Starr’s suspicion that the man was indeed a chimera.

As genetic testing becomes both more advanced and more commercially available, incidents like this may become common. The rise in fertility treatments will likely also contribute to the incidence of chimerism itself, since those procedures often lead to multiple births, or at least multiple fertilized eggs. For obvious reasons, scientists aren’t sure how many of us are chimeras. Only time will tell.

[h/t Buzzfeed]