Unfortunately, the alleles for THC and CBD are physically closely located on the chromosome and hence don't separate like shown in your graph. That's why it's been believed that CBD and THC even share the same allele and it also makes it difficult to breed for and fix certain THC/CBD ratios.
Just received a mail that Dinafem released a Dinamed CBD Auto. Anyone tried that one yet?
. Else, there's really some sort of misunderstanding going on here.
That's exactly what I'm saying!
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I re-read the publication with the genetic map: They published 1.1 cM which corresponds to a hypothetical 1.1% uncoupling/segregation rate although at <7 cM segregation is fairly unlikely. Stupid thing is that there are hotspots where crossovers occur and where not. The chance of segregation derived from breeding experiments is given as LOD value: If I'm reading the graph correctly, the LOD for THCA/CBDA is 40 which means that segregation occurs only in 1 of '1 followed by 40 zeroes' meiosis. Hence, chances to obtain a double homozygous plant either of the BT/BD or Bt/Bd type in the F2 is at 10^40 times 10^40 or simply said never. But all the genes on the chromosome LG6 have values greater than 3 which means that there basically never occurs any crossing over on the whole chromosome! Or I'm reading something wrong (I admit, I never really learned how to used/read gene maps and only did/do it for fun)???
I was referring to something else
At the current state of scientific error, talking of TTdd and ttDD in case of drug type cannabis and hemp, respectively, is correct but using a 4x4 Punnett square isn't. The Td and tD alleles are linked and hence a simple 2x2 square with simplified TD x TD suffices.
THCA and CBDA are of (co-)dominant and not intermediate inheritance (one bell shaped distribution of cannabinoid quantity in the F2 generation, as shown in several publications) and therefore cannabinoid amount is rather due to other traits (available precursors, trichome density etc.). This means that chemotype testing (e.g. chromatography) won't tell genotype ergo distinguish between homo- and heterozygous plants: TTdd and Ttdd will both have THC and no CBD whereas TTDd, TtDd, TtDD, and TTDD all have THC and CBD and that probably at a similar ratio.
Furthermore, your Punnett square not only breaks down to each 3 plants with only THC or CBD and 9 with both cannabinoids (i.e. 1:3:1) but there's also 1 plant with neither THC nor CBD and hence high CBG. That's not what's commonly seen in cross-breeding drug type cannabis with hemp or simply breeding with intermediate types such as charas or hash varieties. Supposedly, there's never been even an indication in any of the germplasm analyses and cross-breeding experiments for the appearance of a double homozygous individual neither of the TTDD nor the ttdd type. Treating the BT and BD loci as one single B locus (i.e. 1:2:1) is closer to reality. Although, none of the publications I've read really hit it spot on. There's always some discrepancy especially the slight shift towards a higher CBD/THC ratio and a loss in overall cannabinoid content but that's the normal discrepancies between biology and statistics.
Taking your CBG variety as an example: chemotype testing won't tell you if it's a ttdd double homozygous plant, a loss of function mutation (either of the BT or BD allele), or the postulated B0 allele. Gene sequencing would be the only way to tell... Talking of which: the Phylos project might already hold the answer to that question.
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