I have observed that genes trump environment. We raised our two children
similarly. However our daughter is a lot like me. Our son is a lot like his mother. If environment mattered more our children should have a blended personality somewhere between the mom and the dad. They don’t. Therefore genes trump environment.

Furthermore genes will express health/illness along with personality. Our son has an allergic tendency like his mother. Given that his personality is similar perhaps the same set of genes express both illness and personality.

Why does this matter? If one looks at the medical history of ones parents/grandparents and perhaps uncles and aunts and correlate their personalities to their children's personalities one can infer that the children have perhaps an 80% chance of developing the same illness and having an 80% chance of responding to the same treatments.
I invite comments.