.More specifically, I got an idea for a story that involves DNA and genetic engineering. But I believe the story would be outdated with modern advances. So, if that's the case, my story would be deemed impossible and implausible.
But if the story is set in a different universe, where discovering the intriques of DNA or something similar hasn't already occurred, how would that be looked upon?
This is all in speculation. I may go and find out the breakthrough in DNA I'm thinking of has yet to actually occur. Maybe I should just write it, mail it and see what editors say, regardless of how out of date the science in my story is...
so, if that's the case, as long as you're internally consistent and the twist actually is integral to the story, go for it.
I have heard complaints that Science Fiction is becoming more difficult to write because developments in science are rendering stories obsolete even before they're printed. I certainly remember one story I read in a collection that had already been overtaken by real life--it was about stem cells, iirc. The story's events depended on an assumption, clearly stated in the text, that had subsequently been proved to be false. It certainly detracted from the story for me, even though it was just unfortunate.
An alternate universe with different laws of physics is tricky, because only within certain parameters can a universe exist at all. Other parameters are essential for stars, planets...life. A lot of research would probably be involved in making your universe viable. And if you hope maybe it doesn't matter, just remember Larry Niven being picketed by people with placards informing him that his Ringworld could not work. It matters .
If it turns out that your idea won't work in a contemporary setting, I don't see any harm in using a Fantasy or alternate setting in which it will work. Try to avoid having it exactly like Earth except it's called Yearth, though 
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Good luck with the idea--sounds interesting.
It's like those old 1920s/30s Gernsbackish stories, where the characters wander through the future, explain how everything works in great and boring detail, and constantly remark on how wonderful it is to live in a world like this. Who acts like that?
Is the only reason you're creating your own universe so you can bend those rules?
Like has been said, science is advancing so fast, that it's difficult to stay ahead of the game. But lets take some past sci-fi writers as examples for us today. I recently read some older stories of Isaac Asimov. Stories written in the 50s that were contemplating far enough into the future that he was thinking about pocket sized computers and a computer-based communications web. I mean, the 50's people! When a computer with the capabilities of today's hand-held calculators was the size of a small house.
So my suggestion would be to expand your learning of today's advancements and the science community's expecations of where we might be 50 years from now, and speculate. Take it just a little further. What might happen next? What might happen as a result? How can I cause meaningful dilemmas for interesting characters that my readers can care about AND understand. It's a tricky thing to do well, IMO. Which is why I don't write sci-fi! 
What I say, is, "backstory should inform the story, but it shouldn't be the story". I'd always recommend to writers that they get their facts right and know what they're writing about.
1. Whatever you decide on the science supporting your story, be logically consistent; and, be certain to carefully provide supporting rules ("science") for anything that is blatantly different from the reader's known universe. Anyone reading SpecFic is already willing to suspend their disbelief. Give them what they need to remain in your universe.
2. Dune, Stranger in a Strange Land, and the other SF classics, have a very specific commonality. They all have excellent characters I came to believe were real, that I cared about, whom I could identify with. The science was secondary, though hypnotically compelling. However, as important as those characters were, had their been a stumble in the known science, I would have been thoroughly disenchanted had that difference not first been established with rigorous foreshadowing. For example, magic is not possible, yet Iain M. Banks, in his Civilization universe, has created a science so far advanced that, for all intents and purposes is magic.
[This message has been edited by Guy Koehler (edited September 12, 2005).]
If you are the master storyteller even Stephen Hawking won't give a flying fudge about your science. But to do that every aspect of your storytelling had better be a thing of beauty. If you show the slightest storytelling weakness, like Tom Clancy or Larry Niven, you had better get the science so good that you send Stephen Hawking back to the drawing board!
Isn't it? I'm asking, because, like I said, I'm not a sci-fi writer. Ask EJS or Survivor what they think.
We are writing on a spectrum from hard SF on one side to high fantasy on the other. If your writing has a SF flavor, but you bend the rules, you are writing science fantasy. Think of Star Trek and you have a prime example of science fantasy. It is NOT science fiction.
It's them that I wonder about, and whether they're necessary. Do I really need to do tedious orbital calculations to ensure that my created planet is the proper distance from its sun? It might be beyond my ability. I only got as far as calculus in school, where I started flunking it and took no more math courses.
If I don't mention it, if I make a planet hot or cold, wet or dry, tropical or glacial, I might get away with that. But do the readers need to know the distance the planet is from the sun?
And how do I write it into the story without the reader saying, "Look! The author is dropping information into his story!"
For example, in a time travel story I wrote, the POV character is talking with the alien:
quote:
"It's really quite simple," he said. "You see, a temporal field is created..." He went on and on about that for ten minutes. I never did figure out what he was talking about.
Enough science to make the technology believable, but not enough to start sounding like I'm spreading the B.S.
So if you're thinking of doing something with DNA that we now definitely know it doesn't do, then you should set this in a fantasy world and probably should call this stuff something other than DNA.
On the other hand, if you're thinking of having someone discover that DNA does something that we don't yet know it does, then you could either set it in the near future or in an "alternate" near past.
Setting it in an alternate near past sorta requires that this be a really important discovery (this is just a "rule" of scale, if you're changing the history of our world you'd better produce more than a mouse).
You can also posit it in a non-alternate near (or even far) past if it is evident from the events of the story that this event would fit into our own past. In this case, the researchers decide to keep it secret or are for some reason unable to publish their results.
If you do it in a near (or far) future setting, then you should push the general state of the art in genetics forward far enough to seem plausible (or create a world in which there has been some decay in science).
Anyway, since you do have other options than splitting the universe off just to tell your story, you probably shouldn't split the universe. Only if your story really is something universe splitting (the cliche example is Hitler getting the Bomb first, or maybe Lee winning at Gettysburg, but Germany did have a Bomb project which ended up being abandoned for a variety of minor reasons). Of course, that's an entirely subjective judgment. So it's up to you