A story began to emerge from the DNA, but it contradicted the one Cline had told his patients. He’d said the donors he used were medical residents. He’d said he used each donor for only three successful pregnancies. But 23andMe showed that he’d used one donor at least eight times, and the birth years of the resulting children ranged from 1979 to 1986. Medical residencies last only a few years. What resident would be around to donate sperm for that entire seven-year period?

The answer, Ballard and her half siblings assumed, was in their DNA. No one in the 23andMe database shared enough DNA with them to be their father, but they found dozens of more distant genetic matches. By combing through public records and social-media profiles, and some times simply asking genetic matches about their families, they could build a giant family tree that, they hoped, would eventually lead to their father. Thousands of adoptees and donor-conceived children have used this method to find their biological parents, and forensic genealogists now use it to investigate cold cases like that of the Golden State Killer.

As Ballard and her half siblings researched their family tree, one suspiciously familiar name kept coming up: Cline. Finally, a woman who shared some of their DNA told them she had a cousin named Donald Cline who was a doctor in Indianapolis.