Probably nowhere near a single gene, but the current state of 'genetic engineering' revolves around searching out keystones (or weak links) to take out or put in and control complex expression.*
It's almost entirely like altering one byte in a binary, but with protein expression instead.
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In other news, makes a good allegory for our concept of 'peace' held since the symbols were developed: the state you hold when not imposing the Powell Doctrine.
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*In other words, castrate someone in Birkenau and you've probably reduced their aggression. Knock out a gene necessary for hormone synthesis and you might achieve the same, of course, 'with some side effects.' We are an annoyingly long way from knowing what the hell we're doing -- in any of medicine, pharmacology, genetics, or software -- to be able to select the 'best' solutions, vs. the only treatments, adjustments, or implementations we know of... but on the other hand, we do know quite a lot, enough to allow a thoughtful person to consider the adequacy or inadequacy of what we've got. (Knock out a gene necessary for tumogenesis with a sole side effect of mild jock itch, and it's something of a no-brainer, at least until pollution from anti-itch cream production becomes a global concern.)
From a 'usability' perspective, we should all just lead pleasurable, free, and immortal lives from the get-go, but that's both thankfully and annoyingly impossible.
