First I would argue that issue of genetics and the chemical imbalance theory aren't necessarily the same thing. Mental illness could be genetic but not be a chemical imbalance. Or it could be a chemical imbalance which doesn't have a genetic cause.

I've given a lot of thought to genetics too. My father was adopted. My family is full of depressives, suicides, and alcoholics in my mom's family and my dad's bio family - but not his adoptive family. That might argue for some kind of genetic mechanism. Or it could be that unhappy people raise unhappy children. The act of giving a child up for adoption is usually a sign that something has gone wrong in the family giving up the child. People who are giving up their children are usually in a lot of pain. And babies do feel the pain of being separated from their birth families - even tiny infants.

IMO there's a lot of evidence that we're born with certain characteristics that can form the core of our personality - excitable vs. placid, bold vs. timid, etc. Then those characteristics interact with our environment to shape us. Some of what we're born with may be genetic but some is environmental. If our mother is stressed while she's pregnant for example, that can effect us.

I think it's so complex it would be hard to tease out what's strictly genetic and what's environment.