> Depression was not the issue according to someone much closer to the situation than you.
So what? Closeness doesn't mean you automatically understand a situation. In fact, an argument could be made that closeness can blind people to objective reasoning about a subject.
> Are his parents depressed? No, I know them both. Were his grandparents depressed? No. I knew his grandfather.
The person you were replying to wasn't implying that his parents are or ever were depressed. He was saying that there is a strong genetic component to depression. Please research "recessive traits".
Let me rephrase your argument to Tay-Sachs disease: "How could he lack the haxosaminidase A protein? His parents did not lack the haxosaminidase A protein, and neither did his grandparents".