Explain howmuscle structure, a gene controlled trait, can be influenced by the environment

The simple answer is - imagine someone who is working out regularly and someone who isn't. 

The person who work out regularly may have more muscles than the other one, but at the same time, he has a worse genetic predisposition for muscle gain. The guy who does not work out on the other hand, has a better genetic predisposition for muscle gain, yet does not work out. 

This is how our environment can influence muscle structure. 

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)