Is this based on one gene controlling both THCA and CBDA production?
That’s what it looks like with pairing of allele and the co-dominant behaviour you are describing.
THCA and CBDA genes are found on chromosome 6 have been found to be two distinct genes.
In your case above you would be assuming both allele from each gene would be dominant and homogeneous; if each gene carried one dominant and one recessive allele the probability becomes less; if the two alleles for each gene are co-dominant this becomes much more complicated.
To be fair there were two theories, one in 2003 (de Meijer et al. 2003) and one in 2006 (Kojoma et al. 2006). It wasn’t until the genome was recently mapped until they found it was two different chromosomes.