Did not read all 8 pages so far, so sorry if this has been mentioned:
In general, I avoid talking about "talent" and prefer to speak of "ability." I've seen far too many people who have this notion that "talent" is just this pre-existing "thing" that you "find" and then BOOM - you're a genius (even if you need to cultivate it a bit). In general, I don't believe in that. I think it encourages way too much laziness and despair ("Why bother, I'm not talented").
I've been reading a fair amount related to psychology and the brain lately, related to my teaching, and it's been interesting. There is a tendency for some people to talk about "genetics," but I've never heard a geneticist identify the "music talent gene." What's getting a lot more attention in current neurology, though, is neuroplasticity. That means that, whatever genetic starting point it has, the brain actually grows and changes, making connections between neurons. This happens especially fast when young (think language acquisition) but to some extent through a person's entire life.
What that means is that by the age a person first is physically capable of manipulating an instrument, their brain has already been forming and morphing based on all kinds of factors, internal and external; but also that WHILE they are having the experience of manipulating an instrument and trying to get music out of it, the brain's neurons are CONSTANTLY zapping and linking and making connections tied to what they're doing, morphing the brain into new configurations around that experience.
So it's probably true, what we observe, that some people seem to just take to an instrument faster and more "naturally" - by whatever means, their brains were primed for it already and the neural links were easy to make from there. It isn't that they were destined from birth by genetics to be great musicians, but it's not just an illusion that they did seem "natural" to it.
There are a few specific things that are useful to musicians that may indeed be purely genetic. Perfect pitch comes to mind, which seems to be you-have-it-or-you-don't, pretty much. You can train your brain to good relative pitch and your memory can retain the pitch of a familiar song pretty exactly (there have been studies done where they compared people singing a song from memory to recordings of the song and they tend to hit pretty close to the recording's pitch - if I type "Just a small-town girl," you're probably hearing Steve Perry's exact notes in your head), but to hear a note without context and know what it is, is an innate gift.
There are also physical things - the timbre of a singer's voice isn't just skill, it's lungs and throat and bone structure, and most of us don't have Pavarotti's sinuses. A pianist with big hands will be able to hit big chords with a wide span more easily (and thus saving effort for more complex matters) than one with small hands. Etc.
