That’s why Mendel’s work was so revolutionary at the time. Everyone had the assumption that all inheritance was incomplete dominance, he was the first to see dominant and recessive traits. If he had seen examples of co-dominance or incomplete dominance in his experiments then he may never had come to his conclusions and followed the status quo 
I’m sure his theories may be applied to other areas as well.
At this point I don’t think anyone has a definitive answer, lots of theories.
There is definitely a gene linking with CBDA and THCA that can be observed. At least my observations are (then again I could be wrong):
THC + CBD <= 26%
High CBD strains at 25:1
High THC strains at 1:25
Equal ratios are <= 10%
Although in strains with THC + CBD < 26%, there could be other cannabinoids at play.