It was Ancestry DNA’s customer-service rep who had to break the news to Catherine St Clair.

For her part, St Clair thought she was inquiring about a technical glitch. Her brother—the brother who along with three other siblings had gifted her the DNA test for her birthday—wasn’t showing up right in her family tree. It was not a glitch, the woman on the line had to explain gently, if this news can ever land gently: The man St Clair thought of as her brother only shared enough DNA with her to be a half-sibling. In fact, she didn’t match any family members on her father’s side. Her biological father must be someone else.

“I looked into a mirror and started crying,” says St Clair, now 56. “I’ve taken for granted my whole life that what I was looking at in the mirror was part my mother and part my dad. And now that half of that person I was looking at in the mirror, I didn’t know who that was.”

St Clair thought she was alone with her loss, and what an odd sort of loss it was. She had grown up in a tight-knit, religious family in Arkansas, never suspecting a thing. Her four older siblings loved her no less as a half-sister. One brother didn’t think it was a big deal at all. “He says he wouldn’t have been this upset if it happened to him,” she told me. “I don’t discuss this topic with him much anymore.” St Clair eventually found her biological father by tracing other matches on Ancestry’s website. He was a stranger her mother knew more than half a century ago. The DNA test didn’t erase her happy childhood memories, but it recast her entire life up to now.