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Yes, in some cases it definitely can be hereditary. But not in all.
For example, my mother is bipolar, as is one of my brothers. Another brother is schizophrenic and I have suffered from depression before. I was able to overcome my depression, and I hope and pray it is never something I can't handle. I have 3 other brothers and I'm not sure how they are doing, we don't really talk. But not all cases of mental illnesses are hereditary.

It also depends on what kind of depression we are talking about, some depression is situational (not hereditary), and you can definitely learn how to be negative, which is a learned behaviour (I know all about this one too). Then there is the depression that is chemical, I'd guess that that is the one that can be hereditary.