what is the genesis of mental illness?

[Guardian]

"there is genetic links there, but typically there are even stronger critical incidents that occur during important developmental milestones that makes a person more likely to develop such disorders.It all depends - how much internal resources you have, resiliency factors, social situations, etc. Plus there are highly individualized aspects of genetics that sort of explains why given high genetic heritability, no symptoms occur"

All of that is epigenetics. Genetics is linear and reductionist, epigenetics is quantum and fully inclusive.

I recommend watching the lecture for a complete explanation of mental illness.