It seems that you are operating under an overly-simplistic consideration of how genes work. There is an entire field called epigenetics which studies gene expression. Having “the gene for something” does not necessarily mean one will have that trait, because the expression of that gene can be altered in a variety of ways. However, that does not imply that the gene is not an important factor in the existence of the trait in those who do display it.
It seems that you are operating under an overly-simplistic consideration of how genes work. There is an entire field called epigenetics which studies gene expression. Having “the gene for something” does not necessarily mean one will have that trait, because the expression of that gene can be altered in a variety of ways. However, that does not imply that the gene is not an important factor in the existence of the trait in those who do display it.