I've heard the concept of "genetic potential" thrown around when referring to bodybuilding. They say that people are hardwired to reach certain size. I have the following questions regarding this elusive concept.

Scientific validity
-------------------

Have scientists singled out the gene that determines the genetic potential a person's muscles? Or is the concept purely based off observation from the bodybuilding community?

Plateau
-------

How can you differentiate between hitting a plateau and hitting your genetic potential? Should a person who's been unsuccessfully trying to break a plateau for over a year give up because it's not really a plateau but rather a genetic brick wall?

Hard Limit?
-----------

Is the limit a hard limit? For example, the gene says one person's upper arm circumference will not exceed 26 inches no matter what. Or is it a ballpark limit? That is, a person will struggle to get past the suggested limit, but will have an exponentially harder time going past that limit.