Very big. Gene is one of the uncontrollable factors. Your personality and your way of thinking depend on uncontrollable factors to a great extent, and uncontrollable factors are still affecting your life all the time. The uncontrollable factors here include genes, environment and various random events. It is impossible to accurately calculate the impact, but there is no doubt that the impact of uncontrollable factors is enormous. The only thing we can do is to try our best to keep our enthusiasm for life regardless of the facts and circumstances. In addition to genes, there is another research field called epigenetics (of course, there are probably other decisive factors in complex issues such as psychological thinking and intelligence). It covers biology, medicine, psychology, sociology and even economics, but then again, at least, I know that some diseases are related to genes, which will further affect personality.