Heterosis, or hybrid vigour, is when the offspring of two genetically different parents performs better than the average of the parents, or better than the best parent, for traits like growth rate, size, yield or stress tolerance. It is the mirror image of inbreeding depression.
In practice
Heterosis is strongest when two distinct, inbred parents are crossed: the F1 is uniform and vigorous. Maize breeding is built on it. In cannabis, growers notice it as F1 seedlings that race away compared with either parent, or a fresh outcross that rescues a tired line.
It fades in the F2. Crossing F1 plants with each other reshuffles the alleles, so part of the vigour is lost and the population becomes variable again. That is why F1 seed is normally remade each time from the parents.
The causes are still debated: masking of deleterious recessives (dominance), favourable allele interactions, and gene-regulation effects all contribute (Schnable and Springer 2013).
Don't confuse with
- Heterozygosity: a genetic state; heterosis is a performance outcome. Heterozygous plants are not automatically vigorous.
- Stretch: fast vertical growth from light or temperature, not hybrid vigour.
- Quality: a vigorous F1 is not necessarily higher in cannabinoids or flavour.
Inbreeding depression is the loss of vigour, fertility, yield or survival that follows mating between relatives, mainly because harmful recessive alleles become homozygous.F1 is the first generation of offspring from crossing two parents; if the parents are inbred and different, F1 plants are uniform and often show hybrid vigour.An outcross is a cross to an unrelated plant from outside the line, used to bring in new traits or vigour, or to rescue a line suffering inbreeding depression.F2 is the second filial generation, made by crossing or selfing F1 plants; it shows the widest segregation, bringing back hidden recessives and new combinations.An allele is one version of a gene; a diploid cannabis plant carries two alleles at each locus, one from each parent, which may be the same or different.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 plant is heterozygous at a locus when it carries two different alleles there (Aa), so its offspring can inherit either one and segregate.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 hybrid is the offspring of two genetically different parents; almost all modern cannabis is hybrid, built from crosses between landrace-derived and other lines.Inbreeding is mating between related plants, which raises homozygosity; an inbred line is the resulting population, measured by the inbreeding coefficient F.
