A quantitative trait is one that varies on a continuous scale rather than in a few clear classes. It is shaped by many genes, each with a small effect, and by the environment. In cannabis, yield, height, internode length, total cannabinoid percentage, terpene content and most of flowering time are quantitative.
In practice
Quantitative traits do not fit Punnett squares. You cannot "fix 25% THC" the way you fix a recessive leaf trait. Instead breeders measure many plants under the same conditions, estimate heritability, and shift the population average a little each generation by selecting the best parents. Progress is steady but slow, and selecting on one trait can drag others with it.
Grow-out size matters. A dozen plants tell you little about a trait whose variation is mostly environmental. Repeat measurements on clones of each candidate separate genotype from luck.
Don't confuse with
- Qualitative (Mendelian) trait: falls into distinct classes controlled by one or a few genes, such as cannabinoid chemotype or autoflowering.
- QTL: the genomic regions that contribute to a quantitative trait.
Myth check
Total cannabinoid content is quantitative and has a biological ceiling. Seed sold as "will hit 40%" is a marketing claim, not genetics; see buyer beware: label claims.
The length of stem between two adjacent nodes; short internodes mean compact plants and tight bud stacking, long ones mean stretch.A cannabinoid is any compound that binds the body's cannabinoid receptors or shares the typical cannabis terpenophenolic structure; the plant-made ones are phytocannabinoids such as THCA and CBDA.Terpenes are volatile hydrocarbon compounds built from five-carbon isoprene units; in cannabis they are made in the trichome heads and are a major part of aroma and flavour.A Punnett square is a grid that lists each parent's possible gametes along two sides to predict the expected genotype ratios among offspring of a cross.A recessive allele only shows its effect when a plant carries two copies of it; in a single copy it is masked by a dominant allele and carried silently.A chemotype is a plant's chemical phenotype, in cannabis usually its dominant cannabinoid class: THC-dominant (I), mixed (II), CBD-dominant (III), CBG-dominant (IV) or cannabinoid-free (V).A QTL (quantitative trait locus) is a region of the genome statistically linked to variation in a quantitative trait such as yield or flowering time.Total cannabinoids is the sum of all cannabinoids a lab measured in a sample, usually reported in % w/w, with acid forms either summed raw or decarboxylation-adjusted.Heritability is the share of a trait's variation within a population that is due to genetic differences rather than environment, on a scale from 0 to 1.Mendelian inheritance is the pattern in which single genes pass from parents to offspring in predictable ratios, following Mendel's laws of segregation and independent assortment.Selection in breeding is choosing which plants become parents, so that alleles behind the chosen traits become more common in the next generation.Genotype by environment interaction (GxE) is when different genotypes respond differently to the same change in conditions, so the best cultivar in one room may not be best in another.
