Which contributes more to developing mental illness: genetics or lifestyle?

I know so little about the development of mental illness. However, it’s prevalent in my family to varying degrees, including immediate family on both sides. In nearly every generation going back to my great-grandparents (the furthest I can reasonably trace back), at least one of their children developed severe mental illness, with multiple diagnoses of schizophrenia.

I’ve existentially wondered if this is a curse, and I’ve also wondered what the biology is behind this. How much of a role do genetics make versus nurture, so to speak?