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A rare case of genetic chimerism changed a paternity test result.After his son was born, genetic testing showed something impossible. He wasn’t the biological father. But there was no mix-up, no mistake, and no infidelity.The answer was even stranger.He was a genetic chimera, meaning his body carried two sets of DNA. Early in development, he had absorbed his unborn twin. Most of his body carried one genetic identity, but some of his reproductive cells carried the DNA of his twin.So when the test was done using cheek cells, it didn’t match his child.Because the DNA that created the child wasn’t technically “his.”It belonged to his twin.Cases like this are extremely rare.Read the study:"A case of chimerism-induced paternity confusion: what ART practitioners can do to prevent future calamity for families." Journal of Assisted Reproduction and Genetics.