Reply to PostWow monk I think you just answered the question of what the symbol is.
It's what a strand of DNA looks like that's been mutated to contain these powers! Think about it, it makes sense! Like you said, it kinda looks like a double helix. Also, we see it on Mohinder's dad's computer when trying to map the DNA or something. This is also why it's on things as simple as a necklace or a pool - it's a piece of art that some people know about, probably from reading Mohinder's dad's books, and so they want to display it. Also, this is why Angela's and Hiro's dad's pictures are marked with it - it is as if to say, "you, who has this special DNA, is marked to be killed".
It might have an even deeper meaning (which is possibly why Takeizo Kensei has it on his flag), but that is I think the basic meaning.
