A Washington man who failed a paternity test for his child discovered a condition called human chimera led to his unborn twin being the child’s genetic father.

Barry Starr, a geneticist at California’s Stanford University, said he and his colleagues studied the 34-year-old man who failed a paternity test after he and his wife discovered their child did not share a blood type with either parent. The fertility clinic that helped the couple become pregnant insisted the man’s semen sample was the one used to impregnate his wife, so the man took a genetic ancestry test, which found the man to be the child’s uncle.

Starr said the result was due to his being a human chimera — the result of the man absorbing cells from a deceased twin during the early stages of their mother’s pregnancy. The man’s sperm was found to be a 10 percent match to the infant, while the genes in his saliva — which were used for the paternity tests — were not a match.

The geneticist said the case marked the first known instance of paternity tests being fooled by a human chimera. “Even geneticists are blown away by this,” Starr told Buzzfeed. Karen Keegan of Boston previously made a similar discovery in 2002 when she discovered her ovaries held genes from an absorbed twin that passed those genes on to two of her sons.