15.4.1 Premise: The Genetics of Chemotype
The cannabinoid chemical composition ofC. sativaplants can be roughly divided
into three main chemotypes, expressing the two most abundant cannabinoids
(THCA and CBDA): chemotype I or“drug type”, characterized by high THCA and
therefore very low CBDA/THCA ratio; chemotype II or“intermediate”, in which
both THCA and CBDA are present at variable concentrations, but with a
CBDA/THCA ratio close to one (0.5–3); and chemotype III or“fiber type”, with a
high CBDA/THCA ratio value, due to the very low THCA content (Small and
Beckstead 1973 ; Fournier and Paris 1980 ; Fournier 1981 ). Two additional, rarer
chemotypes were proposed, chemotype IV, with medium to low content of THCA
and CBDA, but with predominant amounts of their common metabolic precursor,
cannabigerolic acid (CBGA) (Fournier et al. 1987 ; Pacifico et al. 2006 ); and
chemotype V, characterized by undetectable amounts of any cannabinoids
(Virovets 1998 ; Mandolino and Carboni 2004 ). A further chemotype with a high
proportion of cannabichromenic acid (CBCA) (Vogelmann et al. 1988 ; Morimoto
et al. 1997 , 1998 ) has been described (de Meijer et al. 2009 ) (Fig.15.2).
The cannabinoid profile of each plant and its CBDA/THCA ratio is mainly
determined by its genetic background and therefore the chemotype remains quite a
stable feature during the plant life cycle (Fournier et al. 1987 ; Pacifico et al. 2008 ),
though environmental factors do have an impact on the amount of cannabinoid
accumulation (de Meijer et al. 1992 ; Bocsa et al. 1997 ). Thefirst genetic model of
chemotype inheritance inCannabisplants was proposed by de Meijer et al. ( 2003 );
it was based on a single locusB, with two co-dominant alleles (BTandBD) which in
their combinations, determine the three main chemotypes (I, II and III). The
CBG-predominant chemotype IV was tentatively explained by the homozygous
presence of an alleleB 0 with limited or minimal functionality (de Meijer and
Hammond 2005 ), deriving from eitherBDorBT; such interpretation was recently
confirmed upon sequencing THCAS and CBDAS transcripts from this high-CBGA
material (Onofri et al. 2015 ) and identifying specific SNPs in the transcripts at
crucial points of the aminoacidic sequence, putatively altering the full functionality
of the synthases.
As for CBCA-predominant plants, the results of a series of crosses between
accessions with specific characteristics have suggested that the factor responsible
for CBCA proportion is independent from the locusBand has been ascribed to a
separatedfixed locus calledC(de Meijer et al. 2009 ). The sequence of CBCAS has
recently been obtained (J. Stout, personal communication).
15.4.2 Molecular Markers Linked to Chemotype
The single-gene model of the chemotype inheritance, as based on genetic analysis,
prompted the development of several associated markers. One of the earliest,
15 Genomics and Molecular Markers inCannabis sativaL. 331