See Figure 2 from this PNAS paper from J Craig Venter[1]. It caused quite a stir at the time, and not because people thought it was useful or wise.

1 = https://www.pnas.org/doi/full/10.1073/pnas.1711125114

They don't. That's kinda the point. The service takes known genetic markers for things like dark hair, skin tone, bushy eyebrows, freckles. And generates a mockup face with those features.