Where personality is concerned, I think genes are the foundation and certain tendencies are there, but we shouldn’t underestimate just how much environment goes into shaping a person. You can have a pair of identical twins raised under the same roof, under what would seem to be the same conditions, that have all sorts of differences.
Ordinarily, given their 100% genetic match, this wouldn’t make sense. You would expect them to be carbon copies of each other. The difference lies in environment, not the genes. Great example, the gene for Darwin’s tubercle runs in my family (I have one on my left ear). I have identical twin cousins, one of whom had a tubercle on his ear and, interestingly, the other twin didn’t. It was how I used to tell them apart, until he had the thing removed. :)
It could be that one twin gets more nutrients in the womb than the other, or one goes to summer camp and the other doesn’t, or they have different tastes and eat different things. Environment is everything you’ve experienced, from the animate to the inanimate. This is powerful in shaping who we are, both physically and mentally.