Cannabis breeding methods: F1, outcrossing, backcross and IBL.

A strain does not appear by accident — behind it sits a choice of breeding method. Each method solves its own problem: one adds hybrid vigour, another fixes a trait, a third checks how that trait is linked to sex. Let us go through a breeder’s core tools.

Outcrossing and heterosis.

Outcrossing is the crossing of unrelated plants to bring in new genetic material, raise heterosis and reduce the frequency of unwanted traits. Heterosis (hybrid vigour) is the increased viability of offspring that come out stronger than their parents. The first cross of two different parents yields F1, then F2, F3. F1s from homozygous parents are especially prized: they are vigorous and uniform.

Selfing (S1, S2).

Selfing is crossing a plant with itself to preserve its traits. A female plant is induced to produce pollen using silver thiosulfate or colloidal silver. A separate technique is rodelisation — temporarily turning a female plant hermaphrodite by waking a dormant hermaphrodite trait. Generations are labelled S1, S2, S3.

Backcrossing (Bx).

A backcross means crossing a strain with one of its own parents to pull the offspring closer to that parent and fix a trait. Calculated stability by generation:

The textbook example is Cinderella 99 by Brothers Grimm Seeds: the number 99 denotes the percentage of stability; it is a sixth-generation backcross (Bx6). From our own catalogue: Fruit Gum is a Bx5, and Bad Guy, winner of the THC Cup Valencia, is a Bx3.

Inbred lines (IBL).

IBL is the method used to achieve stability within a strain. By carefully selecting the best phenotypes from F1 onwards (usually up to F7–F8), a population with minimal variability is developed. The upside is controlled stabilisation; the downside is a lack of diversity. A finished IBL can be crossed with another IBL to produce a fresh F1 that restores hybrid vigour while keeping the fixed characteristics.

Reciprocal crossing.

A reciprocal cross is a double cross of the same strains with the roles reversed (mother/father). The purpose: to reveal sex-linked traits, to work around problems with weak pollen and to raise the adaptability of the offspring. Importantly, methods can be combined — take an S1 hybrid, cross it with another strain (outcrossing), then stabilise it by backcrossing or IBL.