Dan Vergano at Buzzfeed has an amazing story: a man failed a paternity test for a child that was his. Or should have been his, at least. But the actual DNA in his sperm came not from him, but from an unborn twin brother he reabsorbed in the womb.
No, this is not a weird science fiction novel. It's a rare—but not unknown—medical event called chimerism.
While many pregnancies start out with multiple embryos, often all but one miscarry. But sometimes the DNA from the miscarried siblings is absorbed by the surviving embryo, leaving parts of its DNA in the survivor. Around one in eight pregnancies involves such an event, though most go undetected until a genetic test is performed.
The chimera was discovered after the parents asked their fertility clinic why their newborn son had a blood type different than either of its parents. Concerned that the clinic had used the wrong sperm after the failure of one paternity test, the clinic told them that, in fact, he was the only white donor on the day in question, and their son was Caucasian. A test from the genetics testing service 23andMe showed a stranger result: the son and father were genetically related, but the father was, genetically, his uncle.
Further testing showed that at least 10 percent of the father's sperm came from his reabsorbed brother. As Vergano points out, this isn't unheard of, although it's usually with women failing maternity tests. This appears to be the first time a chimera has sent back a negative paternity test result.