Depression is a sin like being an alcoholic, a smoker, having a drug addiction, being a homosexual etc. I don't believe it's genetic.
There can be hormones or chemical imbalances, that's for sure, but they are caused by a sinful soul.
We can't blame sin on biology (chemical imbalances, problems with the frontal cortex etc.). Psychologists believe that violent murderers and rapists have a problem in the frontal cortex of the brain that causes their violent behaviour, this is all nice and dandy but good luck with using that excuse at judgment day with God. God will not accept a biological predisposition or a medical reason as an excuse for sin. Sin is sickness of the soul, and you can't get to heaven by producing a doctor's certificate documenting your sinful condition (whether a genetic predisposition or a chemical imbalance). Fortunately you can get to heaven by Christ's atonement on the cross if you repent of your sin.
With that said we have to be cautious sometimes condemning depression, because God looks with favour on those that don't have many material posessions or suffer in this world. So if somebody is afflicted by hunger, loss of a relative, persecution, God loves that person and in no way can we say that he will condemn that person if he/she feels a bit down as a result of events like the ones described.