Let's say growing up you may genetically be born with a disease but still be a perfect person as long as the environment you lived in was not harmful. But imagine you were living in a malnourished environment where physical abuse is present and your parents treated you poorly as well. That disease could potentially become your worst nightmare in effect.
Psychologists have come to a conclusion that nature and nurture go almost always hand in hand, which means that you are the person you have become because of the environment you grew up in and because of your heritage. Although this doesn't mean that there aren't other factors, it just means there's a lot that contributes to human development.
This is an interesting concept because in my own experience, my sister and I also are really different people even though we both grew up together but the big difference was that we went to different schools and had a two year age gap in between so we saw things differently. Therefore that had more to do with nurture than nature.