For the genetics vs environment, I think the easiest way to visualize it is as so.
Genetics dictates what is possible for you to do, if you have the genetic code for depression, then it's POSSIBLE (or more likely than normal) that you will get depressed. However, environment plays as the 'activator' for genes, so just because something is possible doesn't mean it'll actually happen unless you have the right environment for it.
An example is like, suppose someone close to you dies, that's an environment that's going to activate some set of genes. Person A might have genes associated to resistance against negative emotions, so he might not get depression from this situation. However person B might have genes that make him less resistant, so therefore he is more likely to get depression. However, if the situation was licking an ice cream, the environment is different so the "depression" gene and the "non-depression" gene don't have any effect.