I've definitely heard about this colloquially with people reacting very differently to the same strain ("we were all fine but she was just freaking out"), and the new science coming out of places like Colorado seems to be showing a strong correlation in that direction. I guess you could say (colloquially again, since I'm not at all in the medical field) that pot can open a bad door in some people that may have stayed shut otherwise. Is that gene expression? Brain plasticity? Dunno.
I think prohibition does way more harm than good and I'm glad to see legalization coming to the states, but I don't think we can treat pot as totally harmless for people under 30 and teens in particular because it seems like it's very harmful for some.
One paper on the subject: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2931552/