Interpretation of double mutants in signaling pathways

[johndoe3344]

I found this in my notes, and I memorized it, but I have no idea why this is. Could anyone shed some light:

Given:
A mutation in A gives repressed reporter expression, while a mutation in B gives constitutive reporter expression.

If:
A double mutation in A and B gives repressed reporter expression... then A positively regulates reporter expression and is negatively regulated by B.

If:
A double mutation in A and B gives constitutive reporter expression... then B negatively regulates reporter expression and is negatively regulated by A.

Why is this the case? Thanks.

 

[Ygggdrasil]

This is an example of a phenomenon called epistasis:
https://learn.genetics.utah.edu/content/pigeons/epistasis/https://www.nature.com/scitable/topicpage/epistasis-gene-interaction-and-phenotype-effects-460