Episode Data's day How did a Romulan manage to evade genetic scans for decades and pass as a Vulcan Federation Ambassador? Almost one hundred years earlier McCoy could spot a Klingon who looked like a human with a simple scan. (TOS The trouble with Tribbles)
I believe the Romulans only left Vulcan about 2000 years prior to TOS so although living on a different planet in a different environment for that period of time, DNA scans shouldn't be able to tell the difference between a Vulcan and a Romulan as they are the same race. Its like Humans for example DNA wise black and white people despite evolving and living in different climates share the same DNA. There's no way other than visually to tell the difference. Although we did get forehead ridges on Romulans and this is only so we the viewer could tell the difference.
Simon Tarses has been passing as part Vulcan his whole career. I don't think there is too much to distinguish between the two
I don't know if Ambassador T'Pell is a totally fake identity or if the Romulans killed or kidnapped the real T'Pell so a spy could impersonate her. If T'Pell was a real person she would have had a real total genome different from that of the Romulan spy. So I guess the Romulan spy would have to be really lucky not to have her genome scanned in enough detail to tell she was not T'Pell. Possibly a 50 year thorough medical examination was scheduled for T'Pell and the fear of being found out caused the fake T'Pell to giver up her spy mission and return to Romulan space. If T'Pell was a fake identity the genetic record of T'Pell would naturally match that of the Romulan spy so nobody would notice any difference. But it would take great skill for people from another planet to create a realistic fake identity. Possibly other Romulan agents already infiltrating Vulcan would be necessary to make T'Pell's fake identity convincing. One of E.E. Smith's Lensman novels, Second Stage Lensman has a plot where Kimball Kinnison infiltrates the military of the capital planet of a a rival power in another galaxy. The natives of the planet happen (or "happen") to be identical with Earth Humans. The intense effort that the Galactic Patrol makes to ensure that Kinnison's fake identity is convincing is described. And unknown to the Galactic Patrol, the Arisians use their greater powers to make Kinninson's fake identity stand up to the even more thorough examination a suspicious Eddorian will give it. I am not certain that the Romulans in TNG have all the relevant abilities that the Galactic Patrol had, let alone Arisian capabilities, so I have to wonder how long the Romulan could pull off impersonating T'Pell.
Yeah, but prolonged separation could make distinctions, as in any evolutionary system. Heck, we never even find out if Romulans have the capacity to mind meld, & that kind of brain differentiation might show a sign or two somehow, maybe. However, to a scan, it could easily not offer much difference
Is this one of those "just go with it to make the plot work" things? It's a good episode, I won't let one little question ruin it. Question we don't know the answer.
Charles Phipps is correct - both are the identical race, and despite having wandered off to another planet, would enough time go by to cause significant genetic changes to the Romulans? Not that a new prequel to try to pawn this off is needed, assuming ENT or DSC didn't try to wring story ideas from this first and TBH some unresolved mysteries are A-OK. That and Robert Stack could have made a third Airplane! movie instead of hosting that show, but I digress... But in the same time period, Chekov stated Vulcan and Romulan physiology are virtually identical ("The Enterprise Incident") but still finds him in a remarkably short period of time. Dramatic throwaway plot element aside, the time in which Romulans and Vulcans separated and the Romulans went to Romulus was sufficient to cause a slight but sufficient variable to be created. There may also be rules and regulations or laws that prohibit 24/7 scanning of every life form wandering around the ship (something DS9 would loosely address via blood scans due to the sub-arc involving Changeling espionage, in part because scanners wouldn't be able to detect a difference - I don't remember if it was ever stated DS9 had the same technologies as the Federation, at the same level of capability...)
Whether "T'Pel" was made out of whole cloth or was an assumed identity, it wouldn't appear impossible for the Romulans to insert the full paper trail required, including dental records and full DNA profile. There are bound to be thick and tall barriers between Vulcan and the rest of the Federation on many issues, certainly including any and all with "personal" in the name! So perverting the records at the source ought to suffice; no need to hunt down DNA scans done by some random starship sawbones, because Vulcans simply don't allow for such things. After all, deep down, Vulcans have a much closer relationship with Romulus than they have with the UFP. Whether they were honest about their seeming ignorance in the 2260s is one thing; their known exchange of information in the 2150s and full knowledge of the Romulans in the 2360s is another. Spies within the Vulcan circles might have plenty of local support, and the elitist and secretive Vulcan diplomats are well known for getting away with pretty much everything... Timo Saloniemi
Also remember the Doc had no problem asking the computer to hide Suder from the scans. I think people are underestimating the amount of genetic change that could happen in 2000 years. It isn’t just a matter of genetic mutation, that would be small. Any genetic factor that influences survival would be affected. And if the original group of Romulan settlers was small and localized there would be major genetic bottlenecking. Add into that, a completely different set of alien DNA would be mixed in. Maybe not enough to be different species, but enough to be as different as different dog breeds.
Spock was half-Human, which may be why Chekov could find him. Spock also did that finger touching thing with the Romulan, which is supposed to be some kind of telepathic kiss for Vulcans. Also, in ENT, the Vulcan Prime Minister(or high commissioner? Leader of the High Command)Vlos(V'los?) was a Romulan. He would have likely had to be there a very long time, so Romulans have been infiltrating and posing as Vulcans for centuries.
Who's Talok? Vlad wanted to return to Romulas at the end of the story, and was told by the other guy that his disappearance would look suspicious.
Whether one (say, Talok) is Romulan by species, birth or merely political alignment is still pretty much up for grabs... - "The Enemy" is our only solid piece of evidence that Romulans as a species are "subtly different" from Vulcans. We don't know what those differences are: the ribosome type compatibility issue that Crusher struggles with and Worf refuses to help out with is unlikely to be one of those differences, as all sorts of species are actually compatible, including Worf's. Rather, it's an individual quirk of the dying Romulan, no different from Sarek finding himself stranded almost without T-negative blood in a ship full of fellow Vulcans. - The ENT Vulcan trilogy is our only, rather vague piece of evidence that Romulans are in fact an old political party or other division on planet Vulcan, merely later famous for having emigrated to another star system. That is, they are the ones "who march under the raptor's wings", and already did before the emigration. - That you'd have to be born on planet Romulus in order to be Romulan is still speculation: we have heard of no other Romulan planet save for Remus, and the extent and nature of their "Star Empire" remains shrouded in mystery. Perhaps PIC will finally help there? Timo Saloniemi
Indeed, the strife that led to the Romulans leaving may have been racial in nature. Perhaps the big war was over forehead ridges? A few of the Romulans would be ridgeless sympathizers for the ridged cause and perhaps also vice versa. Perhaps further phenotypes would have been fought to extinction previously already. Ultimately, race would play a decreasing role in this, as the battle lines would already have been drawn (between star systems, no less). But TNG stands proof to there existing this major division, even if it is just the elephant in the room rather than a valid issue to be settled or ignored. Timo Saloniemi