I don't know about all folk music, but in Western or European folk music tune governs and the chords are at best an afterthought. The older tradition in the United States involved only straight melody or a counterpoint between voice and one or more instruments, or at least I can't think of any exceptions off the top of my head. Chords, strictly speaking come in with guitars in the early 20th Century. In particular, the Carter family played the old songs in barbershop quartet style, which made chords much more important, but hardly as important as the tune.

On the other hand, blues is definitely based on simple chord progressions, and in jazz tunes are commonly improvized over a chord progression, so there you can say the chords govern. Maybe that is an African import; I don't know African music well enough to say