There is almost no mental illness with data that links it to genes. Schizophrenia, for instance, has no data showing a genetic cause and plenty data linking it to environment. Oliver James is lovely to read on this, if you fancy.
I think the general agreement in the field is that genetic abnormalities are necessary but not sufficient to cause Schizophrenia. If you are going to crack, it's usually because you are genetically predisposed to it and in the right environment to trigger the disorder.
Using Schizophrenia as an example, there are plenty of mid-20's males that are in stressful environments...but don't become schizophrenic. A minority of those males will have a genetic abnormality that predisposes them, and they start hearing voices.