Mendelian inheritance is the pattern by which single genes pass to offspring in predictable ratios. Gregor Mendel described it from pea crosses in 1866: each parent passes one of its two alleles per gene at random (segregation), and genes on different chromosomes are inherited independently (independent assortment).
In practice
Mendelian ratios are the breeder's first tool. A single-gene trait with a dominant and a recessive allele gives a 3:1 visible ratio in the F2 and 1:1 in a test cross. Cannabis traits that behave roughly this way include the cannabinoid synthase type (codominant, de Meijer et al. 2003), autoflowering (recessive) and some leaf and colour traits.
Most traits growers care about are not Mendelian. Yield, vigour, resin coverage, total cannabinoid percentage and flowering time are quantitative: many genes of small effect plus the environment. For those, Punnett squares mislead and breeders work with heritability, large populations and repeated selection.
One gene (Aa × Aa): 1 AA : 2 Aa : 1 aa, visible 3:1 if A is dominant. Two independent genes (AaBb × AaBb): visible 9:3:3:1. Test cross (Aa × aa): 1:1.
Don't confuse with
- Quantitative inheritance: continuous variation from many genes.
- Linkage: genes close together on one chromosome break the independent-assortment rule.
A gene is a stretch of DNA that carries the instructions for one functional product, usually a protein such as an enzyme, and is the basic unit of inheritance.Segregation is the separation of a plant's two alleles at each locus into different gametes, so offspring of a heterozygous parent vary in which allele they get.Independent assortment is Mendel's rule that alleles of genes on different chromosomes are passed to gametes independently of each other, creating new trait combinations.A dominant allele is one whose effect shows in the phenotype even when only one copy is present, masking the other (recessive) allele at that locus.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 test cross mates a plant of unknown genotype with one that is homozygous recessive for a trait, so the offspring ratios reveal what the first plant carries.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.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 quantitative trait varies continuously (yield, height, flowering time, cannabinoid content) because it is controlled by many genes of small effect plus the environment.
