> 2) is there any explanation or research on this?

My understanding is that defensiveness is a complex trait that arises 
from several tendencies. For example, tendency to perceive threats, 
tendency to raise alarm, tendency to respond quickly, tendency to sting, 
sensitivity to alarm pheromone, etc. All of these are genetic. If a race 
or breeding line is missing some of these genetic traits (e.g., maybe 
they raise alarm but aren't quick to sting), then the result would be 
docile bees. Another race or line may be missing a different combination 
with the same result, i.e. docile bees. But put them together and you 
can get a full set.

Randall Austin

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