I have 2 genes (t1 and t2), which are affecting a third gene (tg). The response of the t1 and t2 and the tg differs in different conditions. My 3 factors are tf1, tf2, and conditions. 

* If a:tf1, b:tf2, and c:condition, which of the SSs shows if there is a dependency between tf1 and tf2? Is it SSab? 

The reason that I am asking is that I don't see much of a difference in the histogram of the SSab for samples in which there is an interaction between a and b with the samples that there is no interaction between a and b.  

* Also, what is the interpretation of SSabc for my particular problem?