Thinking that a single gene could cause depression isn't fallacious - pinning all depression on it would be. But a single gene as a possible cause isn't surpri

Thinking that a single gene could cause depression isn't fallacious - pinning all depression on it would be. But a single gene as a possible cause isn't surprising given other more extreme single gene effects with other cognitive and developmental effects - angelman syndrome is marked for its cheerfulness and niceness. If you were seeking such a gene you would search for anomalous histories of suicides in a family tree.