Sephiroth gazes into Genesis' eyes, and so Genesis will see the moment (which comes quite quickly) when realization hits, and his hand lifts to Genesis' jaw to turn his head as Sephiroth examines his eyes from different angles. "They're so much greener," he says, fascinated but confused. "Why - ?"

And Genesis will see the moment the explanation hits Sephiroth, too. It's not that hard to put two and two together. And Sephiroth actually turns faintly red. "I...didn't know that was something that could happen."