A friend just asked me about the spectral/spiritual signature I mentioned, and, although I edited my post slightly to hopefully clarify, I'll expound a little bit more here.

When a mutant child is born, it is the genetic mutations that ultimately cause their abilities.  However, the change on the genetic level also (usually) causes a change in a person's spirit, aura, whatever you want to call it, which a handful of people in the world have the ability to see.  This can signal to them that the child will have mutant abilities when they are older (but not what kind).  However, it is still the genetics that cause the abilities, not the spectral signature.