Last week at Buzzfeed News, Dan Vergano described the surprising results of a paternity test — the first known case of a man fathering the son of a brother he didn't know existed. It's an example of a rare genetic condition known as chimerism.
It all comes down to basic elementary-school biology: fraternal twins happen when a woman releases two eggs and they're fertilised by two different sperm. But sometimes, very rarely, those twins fuse early in their development. Only one baby is born, and since that's the normal state of affairs for humans, no one even notices.
These people are called chimeras, and their bodies are built of both their own and their twin's cells. They may never know some of their tissues contain cells that have different genomes. But if the chimerism happens to include their gonads, they can get a bizarre surprise when they have children. Standard paternity or maternity tests will show that their kids aren't theirs.
The Buzzfeed story comes from Barry Starr, one of the geneticists involved with the case. The man's son was born with the help of IVF, but an at-home paternity test showed that the man wasn't the father. The fertility clinic insisted that they'd done the procedure correctly, using his sperm.
That was when the couple approached Starr, who suggested they test the father and son with a direct-to-consumer genetic ancestry test sold by the startup firm 23andMe. The results of those tests came back late last year. Bizarrely, their results said that the man was his son's uncle.
"That was kind of a eureka moment," said Starr. At that point, he realised they might be dealing with a chimera.
Although they're born at the same time, fraternal twins are no more closely related than any other pair of siblings. If some of the vanished twin's cells had built the man's testes, they might also be the source for some of his sperm. Later testing showed that about 10% of the man's sperm carried his brother's genes.