Depression gene 'discovered'

My favorite just-so story about the roots of depression is that it emerged during the last glacial period as a response to extreme cold weather. It's a behavioral adaptation that would have made it easier for humans to remain indoors for long periods of time without going crazy/killing each other.

That's odd, I thought the gene had already been discovered, because I thought I got it from my mom.

It's one of those things I just never bothered questioning, but now I'm confused all of a sudden.