It is mostly not genetic. To the extent it is passed on through families, the environmental aspect of having a parent who suffers from depression is much more important.
Dr. Sorotzkin likes to tell about how he is frequently asked by people looking into a shidduch where the potential shidduch’s parent suffers from a serious mental disorder (so not depression, more like schizophrenia or BPD), whether it is genetic. He wonders why nobody ever thinks to ask what effect it can have on a child environmentally to have a parent who is schizophrenic.