The problem is that much of depression, including who it effects and how badly, isn't caused by genetics. Current research says that there MAY be a genetic connection to depression, but also that a lot of the factors of depression have nothing to do with genetics. Gay kids are 5x more likely to try to kill themselves than straight kids. That's not because gay people are genetically weaker, that's because kids are dicks to gay kids. There is probably a genetic component to depression but it is not so significant that it is the only thing that affects it. Somebody can get severe depression without their parents having it, it is largely a symptom of circumstance.