The truth behind one man's paternity test is mystifying geneticists

By Maria Yagoda
October 27, 2015 04:55 PM

A 34-year-old Washington man and his wife welcomed a healthy baby boy in June 2014 with the help of a fertility clinic. But when they discovered that their baby’s blood didn’t match that of his parents, they decided to take an at-home paternity test.

The results showed that the man’s DNA wasn’t a match, but the couple was even more shocked to discover that the man’s unborn twin is technically the genetic father of their son.

“You can imagine the parents were pretty upset,” Barry Star, a geneticist at Stanford University, told BuzzFeed. “They thought the clinic had used the wrong sperm.”

But the reason why was even more surprising. The father had absorbed the genes of his twin, who died earlier in his mother’s pregnancy, in the womb.

After undergoing a series of tests, the man found that 10 percent of the cells in his semen matched his son’s DNA, which proved he was a chimera – someone with extra genes absorbed from a twin lost in early pregnancy.

“To our knowledge, this is the first reported case in which paternity was initially excluded by standard DNA testing methods and later included as the result of the analysis of different tissues. This case of chimerism yielding a false exclusion is thought to be unusual,” a case study concluded.

This is believed to be the first-ever reported case of a paternity test being affected by a chimera.

“Even geneticists are blown away by this,” said Starr.