A couple had a child with fertility assistance, but later found out that the child’s blood type did not match either parent. A paternity test, using cheek swabs, determined that the man was not the father of the baby. The couple had more tests, and was prepared to sue the fertility clinic. Then they did a genetic test through 23andMe, which tests many more markers than a standard paternity test in order to establish genetic ties in extended families. That test said the man was the baby’s uncle!



The explanation is that the man is a genetic chimera. Before he was born, he had a fraternal twin that did not develop into a viable baby. But the vanished twin’s DNA survived by being absorbed into the surviving twin.

[Stanford University geneticist Barry] Starr reached out to genetic counselor Kayla Sheets of Vibrant Gene Consulting in Cambridge, Massachusetts, earlier this year to confirm the chimerism suspicion, and she in turn recruited Baird’s lab. Cheek swab genetic tests again ruled out the man as the father of his son.



But a test of the man’s semen found out that about 10% of its cells were genetic matches to the infant boy, Baird said. “The sperm showed (the father) was indeed a genetic chimera.”



The father of the boy is effectively the man’s own vanished twin — another ghost —the researchers concluded.

However, since the cells of his lost twin brother are a part of him, he is still the father. The case shows how a person cannot be defined by their DNA. About one in eight single-child births start out as multiple pregnancies, so chimerism is probably more common than we know. Most people have no reason to suspect they have more than one kind of DNA, or to have tests done. But there have been some spectacular cases of chimerism in the news since DNA testing began. Read about them, and about the case described here, at Buzzfeed.