There has been strong epidemiological evidence of the inheritability of the tendency toward depression for at least forty years. In populations all over the world.
I for one don't think one gene is involved. Depression is too variable a condition for one gene to be responsible.
i think you are right. if depression followed the pattern of single-gene inheritance, that fact would be in all the psych textbooks.
also, there are other neurotransmitters besides serotonin that play a role in depression and mood, such as dopamine and norepinephrine.
I guess that makes you perfect, so why are you on this thread?