system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] Why is Genetic Engineering in humans frowned upon, but admitted for human-alien hybrids?

Über-humans like Khan cause a fit of outrage, but human-alien hybrids like Spock - people who need genetic engineering to be produced in a viable way - are seen as ok. Why is the Federation so selective on what is acceptable in terms of genetic modification?
gpt
The Federation is fully accepting of vital life-saving genetic engineering, especially for unborn children who would have a hard time being born, or potentially live a challenging life. Some of the most common medical procedures in the Federation are screenings for genetic disorders, and then distributing gene-therapy cures to make sure they don't crop up later in life. Only the rarest and/or newest problems are incurable, and lead to heartache for everyone involved. 

Hybrid children are very often in need of these vital procedures, and no planet in the Federation would ever stop parents from seeking out treatments to help their child grow up healthy. 

The line is drawn where the engineering goes from *"helping an otherwise disadvantaged person live a healthy lifestyle"* to *"turning a healthy person into a superhuman"* (for lack of a better term) As we've seen time and again, superior ability breeds superior ambition, and basic human nature makes these people start viewing their peers as lessers, if not straight-up inhuman. (us-vs-them tribalism) And when an augmented being starts dehumanizing the "normals", they can do horrific things.

A few people, like Julian Bashir, can resist the temptation, but they're an extreme rarity. The chances of another Khan happening are too great when the idea of engineering becomes a pursuit of superpowers.