Okay, here's where I get really, really obsessive, so please bear with me! The first people through the gate are Sumner, Bates, Weir, Stackhouse, and some random Marine, with a khaki green knapsack. Sumner and the Marine are the only ones wearing a cover. Next through is Ford, also wearing a cover and carrying a black knapsack, then Sheppard. When Sheppard steps through, the lights start coming on. We see Ford (cover, black knapsack) stepping to the side and the lights coming on. But Ford doesn't have the gene. Stackhouse has the gene by "38 Minutes" but we don't know if it's natural or artificial. But even if we assume it's natural and he had the gene in "Rising", it's only until John steps into the city that we see the lights coming on for Ford or anyone else.
Then, John and Rodney start up the steps. We get a shot of John stepping, and the stairs lighting up. A beat later, Rodney comes into view. Weir and Sumner stare at them and Weir asks, "Who's doing that?" and Sumner asks about alien presence. Now, it's not made explicit, but I tend to take all these cues to mean that John's presence is what the city is reacting to. So yeah, it's true he isn't with every Marine recon unit sent out into the city, but his gene has already initialised the city and the others don't have to be gene carriers to make non-essential functions work.
Now, it could very well be that Carson's gene would've been strong enough to wake up the city too, as well as do smaller things like turn on the hologram. We never know because John is the first confirmed gene-carrier to go through and the city did light up. But I think my reading of John as being necessary to wake the city can be supported by canon, which is why I don't have a problem with fics that use this particular fanon. YMMV, yadda, yadda. *g* Heh. Told you I was obsessive!
Re: part 2
Then, John and Rodney start up the steps. We get a shot of John stepping, and the stairs lighting up. A beat later, Rodney comes into view. Weir and Sumner stare at them and Weir asks, "Who's doing that?" and Sumner asks about alien presence. Now, it's not made explicit, but I tend to take all these cues to mean that John's presence is what the city is reacting to. So yeah, it's true he isn't with every Marine recon unit sent out into the city, but his gene has already initialised the city and the others don't have to be gene carriers to make non-essential functions work.
Now, it could very well be that Carson's gene would've been strong enough to wake up the city too, as well as do smaller things like turn on the hologram. We never know because John is the first confirmed gene-carrier to go through and the city did light up. But I think my reading of John as being necessary to wake the city can be supported by canon, which is why I don't have a problem with fics that use this particular fanon. YMMV, yadda, yadda. *g* Heh. Told you I was obsessive!