Actually, the "one gene, one behavior" hypothesis is fairly outdated, it's pretty well recognized that behavioral genetics are more complicated than that, with multiple genes influencing a particular behavior, and multiple behaviors affected by a single gene.
Now, if you read about something like a "gene for aggression," it really means, "one of genes for aggression", and does not exclude that it might also be involved in other behaviors, such as fearfulness, maternal behavior, etc.