I can't remember, how long after TNG does DS9 take place?

They actually overlap. DS9 started about halfway through TNG season 6. And the 2-part TNG episode "Birthright" in season 6 takes place in part on the DS9 station. But the thing is, when they're later dealing with Bashir's background and the problems with his genetic enhancements, there's no indication that it's a recent phenomenon. And his genetic enhancements were done when he was a child, which would have predated the start of TNG by several years. Bashir might have been in medical school and still/already hiding his genetic enhancements at the same time the Darwin Station was doing their experiments.

@Knixon said:

DS9 showed us that genetic engineering is forbidden in the Federation, at least for humans, and yet the TNG episode "Unnatural Selection" showed that it was being done as part of officially recognized research.

I wonder why?

As far as I can recall, it was in the DS9 S5E19 episode "Doctor Bashir, I Presume (1997)" that it was first mentioned that the Federation had made genetic engineering for any reason other than repairing serious birth defects illegal.

Why hadn't other writers thought of including it in earlier episodes when there was any talk about genetic engineering?

The (S)TNG episode "Unnatural Selection (1989)" appeared eight years earlier, and this 'ban' was not mentioned there. Nor was it ever mentioned in any earlier Star Trek episodes or movies. The impression I got when I watched that (S)TNG episode for the first time was that research in genetic engineering was permitted and used for medical reasons.

The inconsistency is easily explained by the fact that the writers of that DS9 episode decided then that, for dramatic reasons and to up the ante, it would be convenient for the plot if genetic engineering was banned in the Federation after The Eugenics Wars. A story about Bashir's possible expulsion from Starfleet and his father's punishment because of that was more interesting than just the drama of a parent-son relationship and the shame of knowing that he was genetically enhanced because he was an average human child.

This 'ban'—first mentioned by the writers in "Doctor Bashir, I Presume (1997)"—affected all the Star Trek episodes before 1997 retroactively, and now that this Federation policy is established as fact since 1997, everyone is scrambling to explain and rationalise why there was no mention of any ban on genetic engineering in earlier episodes.

The writers of the DS9 episode did have some good reasoning about making genetic engineering illegal in the Federation. It is just that those before them should have come up with it the first time when the augmented humans appeared ((S)TOS "Space Seed (S1E22, 1967)").

The idea to ban something—including any research of it—that could cause harm to the human species reminds me of "The Omega Directive", which goes much further, including actively implementing that policy and disregarding even the Prime Directive. By the way, that story is from the same writer as the above-mentioned DS9 episode: Jimmy Diggs.

The short version: "Because Plot/Because Drama."

Regarding the Omega Directive, that exception might be made because no civilization would be able to have anything to do with Omega using "stone knives and bearskins."

As Kirk once said:

KIRK: No. The people of Yonada may be changed by the knowledge, but it's better than exterminating them.

SPOCK: Logical, Captain.

KIRK: And the three billion on Daran Five.

SPOCK: Also logical, Captain.

Indeed, the planet involved had rather advanced warp drive. They were able to pursue and overtake Voyager.

It also would have - and did, in that episode - endanger the host civilization itself, as well as everyone else.

It's also possible that the genetics ban was in effect the whole time, I don't recall any other episode that might have addressed it; but the writer(s) of the "Unnatural Selection" episode used the Because Plot/Because Drama excuse, in part because they wanted to show how those people created telepathic children that moved chess pieces with their mind, etc.

@Knixon said:

Regarding the Omega Directive, that exception might be made because no civilization would be able to have anything to do with Omega using "stone knives and bearskins."

As Kirk once said:

KIRK: No. The people of Yonada may be changed by the knowledge, but it's better than exterminating them.

SPOCK: Logical, Captain.

KIRK: And the three billion on Daran Five.

SPOCK: Also logical, Captain.

Indeed, the planet involved had rather advanced warp drive. They were able to pursue and overtake Voyager.

It also would have - and did, in that episode - endanger the host civilization itself, as well as everyone else.

It's too bad that Janeway destroyed all the data that Seven collected about the Omega particle. She succeeded in stabilizing it for at least 3.2 seconds until Janeway blew it up. That is a huge improvement from the one trillionth of a nanosecond that the Borg Collective achieved.

Probably because it was still an incredible risk. But I don't believe the claim that a single Omega molecule contains the energy equivalent to a warp core. That's just a plot device, to get them involved, in a way different from what happened with the episode "Time And Again," which was only the second regular episode following the pilot.

It was also kinda ridiculous to claim they could use the transporter on it.

@Knixon said:

Probably because it was still an incredible risk. But I don't believe the claim that a single Omega molecule contains the energy equivalent to a warp core. That's just a plot device, to get them involved, in a way different from what happened with the episode "Time And Again," which was only the second regular episode following the pilot.

It was also kinda ridiculous to claim they could use the transporter on it.

The transporter is always there to save the day. Yes, also in "Unnatural Selection". .

@wonder2wonder said:

@Knixon said:

Probably because it was still an incredible risk. But I don't believe the claim that a single Omega molecule contains the energy equivalent to a warp core. That's just a plot device, to get them involved, in a way different from what happened with the episode "Time And Again," which was only the second regular episode following the pilot.

It was also kinda ridiculous to claim they could use the transporter on it.

The transporter is always there to save the day. Yes, also in "Unnatural Selection". .

I usually figure the Animated Series as having been really the worst for what I call "Transporter Magic."