Because the incorporation happened in utero, presumably early in development, the immune system was trained to see both the man’s DNA and his twin brother’s as self.ĭieter Egli, a stem cell scientist at Columbia University, notes that chimerism is what occurs with organ transplants, and, on a more sophisticated level, with the new stem cell-based treatments that researchers are exploring now. But some cells with differing DNA can be folded into a developing fetus and not be rejected because the fetus’ immune system isn’t developed enough to see it as foreign. It’s not that an entirely new genome can be subsumed into another one, which would result in an excess of genetic material that is lethal. It’s known that germ cells that go on to make egg and sperm are also quite mobile and do migrate: in this man’s case, apparently from his unborn twin to him. Some cells outside of a developing fetus-in this case, the man’s fraternal twin (not identical, since their DNAs are different)-manage to get incorporated during the chaotic series of cell division and proliferation that occurs when an embryo starts to grow. These can migrate throughout the body and have been found in the lungs, thyroid, muscle, blood, heart and even the brain of the mother.īut what makes this man’s case unusual is that some of the cells he obtained from his twin were apparently germline cells, which develop into eggs or sperm. Mothers retain some cells of their children, and a recent Danish study found that women who gave birth to boys retained cells with Y chromosomes. Previous cases of chimeras revealed people with cells that contained DNA different from their own. Such cases are exceedingly rare, but they do occur. The man’s previous child’s DNA matched his, so, as Starr says, “He is like dad and uncle to his kids.” Apparently, the father had absorbed some of his twin’s cells in the womb, effectively becoming a blend, or chimera, of himself and his brother. Vanishing twin syndrome, which refers to the condition in which one twin dies and is “absorbed” by the other, or by the mother or the placenta, occurs in anywhere from 20% to 30% of pregnancies with multiple babies. It turned out that the DNA in the man’s sperm, which was 90% his DNA and 10% that of his twin’s, was from his unborn fraternal twin. If it is an uncle to a niece or nephew, it’s 25% related. If it was a parent-child relationship, you would see 50% of the DNA related.
When the report arrived, Starr interpreted it for them and, he says, “it just leapt out at me: uncle. The consumer-based genetic testing company provides more detailed relationship and ancestry type genetic data-using hundreds of thousands of markers on the genome-than the dozen or so markers that paternity tests cover. Starr, director of outreach activities at Stanford University’s Department of Genetics, suggested that the couple get a 23andMe analysis done. The couple hired a lawyer, who wrote in to Barry Starr’s Ask a Geneticist blog.