Well. I think you might be getting a little bit too eager with your reductionist approach to mental illness. Sure there is a large genetic component to schizophrenia - I was terrified for years that I might have it because my mother did. I think it more likely that shizophrenia is a heterogenous illness - there are many pathways to the same disease state. It is quite a bold statement to say that none of these pathways are in any way effected by the environment in which we develop. Especially given what we know about the association between early traumatic experiences and various disease states.
It may not fit nicely into your mental model of the world, but biology is extremely messy business. It appears that biological systems, although heirachical and modular are, unlike human engineering, highly interconnected across all the levels. Particularly: The health of the mind effects the health of the body and the health of the body effects the health of the mind. If you do not believe this you have not learnt enough, or like many, you have ideoligical issues with this reality.