Hemp genetics and breeding matter. The proliferation of high-profile seed scams—with seeds that produce “hot,” poor quality, or unfeminized crops—illustrates the point. A recent study published in Global Change Biology-Bioenergy confirms the role that genetics play in the success of a hemp harvest. 
Farmers often blame environmental stress when crops exceed the legal threshold (0.3%) of tetrahydrocannabinol (THC). This reasoning is not ungrounded. Environmental stress has been shown to increase THC in intoxicating cannabis varieties and increase CBD in hemp varieties. However, according to the new research, the genetics of the plant also help determine how much THC the hemp produces. 
Larry Smart, professor at Cornell University, and his research team (Toth et al ) cultivated over 200 hemp plants (14 varieties) at two different sites in New York then analyzed their genetics and chemistry. Incidentally, one of the locations suffered natural flood stress during flowering.
They categorized female plants genetically based on their combination of alleles (variant genes) coding for either tetrahydrocannabinolic acid (THCA) synthase or cannabidiolic acid (CBDA) synthase—the alleles are known as BT and BD, respectively. The synthase enzymes catalyze formation of their respective cannabinoid acids (the acidic forms of THC and CBD). In the study, the hemp plants lined up as follows:
- THC-dominant alleles (BT/BT): 2 plants
- Heterozygous (BT/BD): 65 plants
- Hemp-dominant alleles (BD/BD): 150 plants
The field locations did not make a significant difference in cannabinoid content. The authors noted that “cultivar was the best predictor of total potential cannabinoid concentration.” All BD/BD plants were below the 0.3% threshold for delta-9-THC, but only 35% of the heterozygous plants passed the test. Interestingly, only 39% of the BD/BD plants were below 0.3% total potential THC (due to high general cannabinoid production). The researchers discovered that “[c]ultivar populations that were thought to be stabilized for CBD production were found to be segregating phenotypically and genotypically.” Genetics were strongly correlated to cannabinoid ratio (CBD:THC) and total THC. The researchers also speculated that “differences in cannabinoid production ascribed to changes in environment may in fact be due to sampling of individual plants with BT alleles.” 
Thus, with hemp, farmers have to look at nature and nurture. Any plant with an allele that promotes THC synthesis is likely to cause issues.
- Toth, J. A., et al. (2020). Development and validation of genetic markers for sex and cannabinoid chemotype in Cannabis sativa GCB Bioenergy.2020, doi:10.1111/gcbb.12667.