Yeah, that was the premise of Days of Future Past, the Sentinels realizing that 'the mutant problem' required them to address the 90-ish percent of people who weren't mutants, but could still have mutant children (or get zapped by cosmic rays or gamma rays or bitten by a radioactive spider and become 'mutates' with super-powers like the Hulk, Fantastic Four or Spider-Man).
What's never really been satisfactorily explored, IMO, is that there are still some few humans who *don't* have a latent X-gene, whom the Sentinels of Days of Future Past regarded as 'pure.' That's kind of amazing, that enough original (pre-Celestial tinkering) human DNA is still floating around that you can have modern day humans who just crap out their intestines and die if exposed to gamma rays, instead of turning green and getting super-powers.
Also the location of the X-gene matters. It can't *just* be on the Y chromosome, or there'd be no X-ladies, it's literally be the X-MEN. But if it's then just on the X chromosome, then it suggests that there can be women with *TWO* copies of the X-gene, and therefore, in theory, X-ladies with two completely different mutations. (Jean and Emma with their multiple powersets seem like good examples of such, but there are plenty of X-dudes who seem to have multiple powers, like Cable, Nathan Summers, Quentin Quire, even perhaps Nightcrawler, with unrelated physical mutations *and* teleportation.)
It seems like the X-gene can manifest on *either* chromosome.
But that's a shame. A part of me likes the idea that only women can be rare 'double mutants.' I think Claremont would have loved it, back in the days when Rogue, Storm, Dazzler and Psylocke were the heavy-hitter X-'Men'.