Notwithstanding the particulars about serotonin, I'm not disputing that brain chemistry balances might be heritable to some extent, like height. But "depression" is a downstream phenomenon of one's brain chemistry interacting with their ecology.
Here's an analogy: If I took Nordic people and gradually shrank the architecture, chairs, and desks around them across a few generations, such that most would have to unnatural bowing of the head and curvature of the spine, they would experience enormous agony - back pains, neck pains, etc (like "depression"). They would even correctly identify that they are suffering due to a genetically inherited length of their skeleton. Now imagine that in such a world, the "medical solution" is to remove a couple of vertebrae to make them shorter or take some pills that anesthetize the neck muscles. Let's even say all that happens without side effects. Many a patient would be happy for the agony to go away.
And yet, I still think this is completely the wrong approach. I have deep compassion for depressed people. I believe they should be very careful trusting a system that "shrunk the architecture" so to speak and offers a form of brain chemistry interference as a "solution".
In my opinion, depressed people should attempt to remember that it is almost mathematically impossible that they are the descendants of dozens of generations of depressed adults. They would be far better served, in my opinion, to try to recall and recreate the ancestral adaptive environment (the bigger chairs and taller ceilings).