Quote Originally Posted by Heero
is FFX related to our world? no
Quote Originally Posted by Heero
besides, the chromosomes in ur genes create ur hair color and the follicles of ur hair have those genes in them, so if they are exposed to the sun long enough, they can be altered to where the dominant traits(dark hair color) are weakening to recessive traits(light hair color).
Tidus is (SPOILER)a dream of the Fayth, and as such I don't think chromosomes and the like have much meaning. As you said, Spira does not conform to the norms of reality. His hair can be whatever (SPOILER)the Fayth want it to be. I wouldn't use science to justify something in a video game.

Then again, it could be possible that Tidus could choosse to alter his appearence, much like (SPOILER)Auron.