Sorry sir, you're not the father of your newborn child—your unborn brother is. So a 34-year-old man was told in the only known case of a paternity test being tricked by a so-called "human chimera," the Independent reports.

It began when a US couple learned that their son, born healthy in 2014 with the aid of a fertility clinic, had a different blood type than his parents.

Concerned, the husband took an at-home paternity test and learned that his DNA didn't match the son's. "You can imagine the parents were pretty upset," Stanford University geneticist Barry Starr tells BuzzFeed.

"They thought the clinic had used the wrong sperm." The clinic assured them there was no mixup, so the couple got a lawyer and approached Starr for help.

On Starr's advice, the father and son took DNA ancestry tests and learned that genetically the dad was his son's uncle. In other words, the dad was a "human chimera" who had soaked up the genes of a twin that perished in the womb, then passed them on to the son.

Starr saw further evidence of chimerism in the father's skin (which was two-toned in dark and light strips) and a DNA test of his semen, which identified a slight father-son genetic match of 10 percent.

Human chimerism isn't new, though: It's been seen on TV shows like CSI and in real-life cases, like a mom who took a DNA test for a kidney transplant and learned she wasn't the biological mother of two of her sons, or another woman who got similar news after submitting a DNA sample for welfare payments, the Daily Beast reports.

"Human chimerism is very common, but exquisitely difficult to identify, coming to light almost exclusively by accidents like this," a biologist says. (DNA tests have also opened more Shroud of Turin mysteries.)

This article originally appeared on Newser: Dad Learns That Unborn Twin 'Fathered' His Son

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