When Beatrix was born, the obstetrician harvested blood rich in stem cells from her umbilical cord.

Using a revolutionary laboratory technique, researchers used these stem cells to make an exact genetic clone of her—with one difference. By altering the DMRT1 gene on the ninth chromosome, they made the clone male. Gene for gene, Beatrix and this clone were otherwise biologically identical. Only their sexes were reversed.

When the two of them finally met twenty years later, the similarities between them were striking. The tendency to muse during lulls in conversation—the dislike of crowds or loud public spaces—the almost pathological antipathy toward doing uninteresting tasks...

That he was ten months her junior didn't bother her. Beatrix assumed she'd found her soul mate.

He left her once the facts came out. He had always wanted kids—but any child conceived between them would not just be inbred; it would be self-inbred. "God, how creepy," had been his last words to her on the matter.

This was three years ago. Now she walks as one dead, hollowed out by pain and loneliness. If only I'd never known him, she thinks.