It's never made much sense that the entire multi-species Federation would be subject to a strict ban on genetic engineering due to events on Earth that happened centuries before the Federation was even founded. The way they doubled down on that rationale in Una's trial only highlighted the absurdity -- especially when Admiral April claimed he would exclude Una to prevent genocide.
On the one hand, the writers may be trying to create a straw man out of a weird part of Star Trek lore so they can have a civil rights issue in Starfleet. And that's fine. From an in-universe perspective, though, I think we can discern another reason for the ban on genetic engineering -- the Klingon Augment Virus.
There was a ban on genetic engineering on United Earth, which is understandable given that it was much closer to the time of the Eugenics Wars. Why would that remain unchanged when more time passed, more species joined, and more humans lived in places without living reminders of the war? [NOTE: I have updated the paragraph up to this point to reflect @Value Subtracted's correction in comments.] The answer is presumably that they needed to reassure the Klingons that something like the Augment Virus would never happen again. Hence they instituted a blanket ban around that time -- perhaps in 2155, the year after the Klingon Augment Virus crisis and also, according to Michael Burnham, the year the Geneva Protocols on Biological Weapons were updated.
That bought the Federation over a century of peace, but after war broke out due to a paranoid faction of Klingons who thought humans would dilute Klingon purity and after peace was only secured through the most improbable means, they doubled down on the ban. Una's revelation provided a perfect opportunity to signal to the Klingons that they were serious about the ban -- hence why they would add the charges of sedition, perhaps. Ultimately, an infinitely long speech and the prospect of losing one of their best captains combined to make them find a loophole -- but not to invalidate the ban or call it into question. This Klingon context is why April, who we know is caught up in war planning of various kinds, is so passionate that the ban exists "to prevent genocide" -- he's not thinking of people like Una, he's thinking of the near-genocide they suffered at the hands of the Klingons.
This theory still doesn't paint the Federation in a positive light, since they have effectively invented a false propaganda story to defend a policy that has led to demonstrable harm. But it makes a little more sense, at least to me. What do you think?
On the other hand, historically, attempts at a cure have also resulted in rather terrible treatment for disabled people, particularly where eugenics tended to rear its ugly head.
And from a slightly tangential viewpoint, some of the reaction is also similar to the Deaf community's reaction to the possibility of a cure. Working around their disability has caused them to form a culture of their own, so in their eyes, the existence of a cure is an implicit threat of genocide of some form.
Besides it being anecdotal and not applicable to everyone, you also run the risk of the Bashir issue, where there's the possibility that someone would be just fine growing up (at least going by Mirror Bashir), but their parents try to get them bumped up because they're not doing as well as they would like, or they're pressured into it just by virtue of being different.
The problem is where you draw the line. Even with just "disease and disorders", that is fantastically broad category, and is very much up to interpretation.
It wasn't all that long ago that being attracted to members of the same sex was considered a disorder of sexual deviancy, on par with paedophilia, and there some conditions are arguably benign enough that they don't warrant that kind of treatment.
The modern interpretation is generally that if the patient's quality of life is not affected, then they don't really need a medical procedure done. Someone whose organs are backwards, or has a single fused kidney instead of two, for example, doesn't need to undergo an immensely complex surgery to have them put in their common places/reshaped, since that arrangement doesn't cause them any issues.
You could argue the same for neurodevelopmental disorders. If the patient's quality of life is severely harmed, then yes, treatment might be something worth considering, but you also don't want to blanket apply that across the board.
That aside, I'm also not sure that GATTACA is quite the best example to choose, considering that in that universe, genetically designed/augmented people were both preferred, and the only ones allowed to take many roles, creating a permanent underclass of naturally-born people who really only qualified for janitorial jobs, and things no-one else wanted to take.
I don't want to be thought of as rude, but it's incredibly insulting to use the word "genocide" to describe, say, cochlear implants.
During the 1930s and 1940s, Hitler and Stalin didn't force Jews, gay people, etc. to convert to Christianity; they were killed for being "wrong". An entire people were nearly wiped out via mass murder.
That's what genocide is. MASS MURDER.
You're comparing possible genetic therapy to allow people to blend in, to !!!MASS MURDER!!!
EDIT: I thought of a better analogy: people who are paraplegic due to spinal injury, and stem cell research. There's promising research into curing some spinal injuries previously untreatable, which would allow people to walk again. The argument many disabled communities make, is that doing so deprives that person, and the world, of a different life experience where they can live a meaningful life in a wheelchair, and that treating their injury is no different than throwing them into an oven at a Nazi mercy center. It leaves out entirely the wishes of the individual, suggesting it's more important to be a visible disabled person than their personal wishes.
Suggesting that, say, your child could avoid having to learn coping mechanisms to deal with ADHD by never having ADHD in the first place, isn't murder. Now, testing your fetus and aborting it because it's going to be disabled, on the other hand...
It is how portions of the Deaf community have described it. For example, some portions believed that by giving deaf children cochlear implants, and through them, the ability to hear, they would be removed from the Deaf community, which they could consider genocide (see definition 2 quoted replies below).
That article isn't the only one that mentions that, either. There are others, both in the form of blog posts, or research article.
It may surprise you to know that neither Hitler, nor Stalin actually supported religion at all (and is probably where the contemporary US attitude of "we're proud Christian Americans instead of those communist atheists" comes from, but I digress.
For Nazi Germany, the murder was a more convenient way of achieving their goal of elevating Germans through eugenics by eliminating "undesirables", but they did also sterilise them too.
For Stalin's USSR, they also exiled people in addition to executing them. I can't say why, but I imagine it was also a matter of convenience compared to exile.
Genocide need not involve murder. You can also have a cultural genocide, where you aren't trying to erase a peoples by killing them all, but using other means, such as forcing them to lose their language/heritage, or taking away their children. Australia did that back in the colonial days, and it would be difficult to argue that it was not an attempt at genocide.
You do make good points, since the line that's typically drawn in medicine is one of quality of life. If a patient's quality of life is impacted by their disability, then it would make sense to offer the option of a treatment.
Raphael Lemkin, writing in 1944 in the very paper that first established the term "genocide", wrote the following:
Maybe you should make sure you're right about terminology before you embarass yourself with weird histrionics?