Hi
Mental illness, or at least depression, is only partially genetic. it's more like the genetics give you a predisposition for it, but the environmental trigger is still necessary for it to actually show up. my mom had depression and for most of my childhood received either no treatment or inadequate treatment (i say inadequate simply because of the way she acted towards me & my brother). i think it is that exposure to a volatile upbringing that made me so susceptible to depression as an adult.
the heritability of depression didn't really occur to me when i got pregnant, possibly because i believe that environment is a much stronger component than genetics. if nothing else, and if the worst happens and my daughter does end up with depression, at least she will have in me someone who knows exactly what she is going through, someone who won't respond to her feelings with "it isn't that bad," or "just snap out of it," or "fake it till you make it" or the myriad other lines of crap I've gotten from people (including my own mom).