My own view is that there is no one major influence in a person’s life. Instead, the traits we inherit from our parents and the situations and experiences that we encounter in life are constantly interacting. It is the interaction of the two that shapes a person’s personality and dictates how that personality develops. If this were not true, then we would be able to predict the behaviour and character of a person from the moment they were born.
In conclusion, I do not think that either nature or nurture is the major influence on a person, but that both have powerful effects. How these factors interact is still unknown today and they remain largely unpredictable in a person’s life.