Thinking of buying a DNA testing kit for Christmas? You may find out more than you bargained for…

As a story-loving child more likely to be found playing detectives than the now-suspect doctors and nurses, I yearned for a family secret. My parents had both been raised with them: in my mother’s case, her dad’s Jewishness was kept hidden from her; in my father’s, paternity remained an unsolved mystery (Pétainist French Catholic priest or local milkman?).

But I wanted my own, preferably one that, hewing to the family theme,…