Greg Cox wrote:
(And Timo is right; even the idea that Khan and his followers were genetically-engineered is something of a retcon. "Space Seed" only mentions selective breeding.)
Well, Kirk does mention "controlled genetics."
KIRK: I'd like those answers now. First, the purpose of your star flight.
KHAN: A new life, a chance to build a world. Other things I doubt you would understand.
KIRK: Why? Because I'm not a product of controlled genetics?
That's right, but for some reason his words with Spock about selective breeding were a lot more memorable.
Anyway, "controlled genetics" could mean something as simple as selective breeding if we wanted to read it that way.