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Breeding · Breeder · 3 min read

Backcrossing, IBLs and stabilisation

A backcross crosses offspring back to one parent (the recurrent parent) to recover that parent's genome while keeping a trait from the other: each backcross halves the donor's share (50%, 25%, 12.5%, 6.25%...). An IBL is a line inbred and selected over many generations until it breeds fairly true. 'Stable' should mean measured uniformity across a grow-out, not a generation count.

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Short answer. Backcrossing moves a trait into a line you already like: cross, then keep crossing selected offspring back to the line you want to recover. After three backcrosses the offspring average ~94% recurrent-parent genome. Stabilisation is different: it means narrowing a population, by inbreeding and selection, until it produces the same type reliably. Neither is finished until you measure uniformity in a real grow-out.

Backcrossing

The mechanics

  • Donor = carries the trait you want (e.g., autoflowering, CBD, mould resistance).
  • Recurrent parent = the line you want to end up with.
  • F1 = donor x recurrent. BX1 = F1 x recurrent. BX2 = selected BX1 x recurrent. And so on.
Generation Average recurrent-parent genome Donor genome
F1 50% 50%
BX1 75% 25%
BX2 87.5% 12.5%
BX3 93.75% 6.25%
BX4 96.9% 3.1%

These are averages [1]. Real plants vary, and DNA from around the target gene ("linkage drag") stays longer, especially in low-recombination regions of the cannabis genome [6].

Dominant vs recessive targets

  • Dominant trait: select carriers each generation by eye, then backcross again.
  • Recessive trait (e.g., autoflowering [4]): BX offspring are carriers but don't show it. Either self or sib-cross a generation to reveal it, then continue, or use a DNA marker to pick carriers.
  • Codominant trait (chemotype [3]): visible by lab test in every generation.

"Cubing" in cannabis

Grower jargon for repeatedly backcrossing to one clone (the clone is used as the recurrent parent three times: "BX3" = "cubed"). It recovers a lot of the clone's genome, but the progeny are still segregating and inbred towards one individual; it is not a copy of the clone (Grower practice term; genetics Established).

Stabilisation and IBLs

What "stable" should mean

A line is stable for a trait when grow-outs reproduce it at a stated frequency. The useful definitions come from variety testing: Distinct, Uniform, Stable (DUS). Uniformity means off-types stay below a set threshold in a sample, and stability means the line still looks the same after repeated propagation [5].

How to get there

  1. Start with a large selected population (not one pair).
  2. Each generation, grow enough plants to see the variation (≥20-50 females where possible).
  3. Select for the target traits; keep several parents (2-5 females, 2-5 males) to limit inbreeding depression [2].
  4. Track the off-type rate per trait per generation.
  5. Stop calling it "stabilising" and start calling it stable when the off-type rate has stayed below your published threshold for at least two generations and two environments.

IBL claims

"IBL" has no fixed definition in cannabis. Treat it as a breeder claim (L2 on GenetixBay) unless supported by published grow-out data (uniformity tables, tester rounds) or genomic data showing low heterozygosity (L3-L4).

Numbers that matter

Measure Example target How to measure
Off-type rate for a key trait ≤5% (breeder-set; DUS uses formal statistical thresholds) Count in a grow-out of ≥50 plants
Intersex rate ≤2% (breeder-set); intersex plants are culled, never used as parents All plants observed to harvest
Flowering time spread ±7 days Days flip to harvest
Chemotype off-types (for a type III line) 0 in ≥30 tested Lab or marker test

Targets are suggested GenetixBay practice, not legal standards.

Common mistakes

  • Backcrossing to a clone with a hidden intersex tendency: you concentrate it. Prove the recurrent parent sex-stable over at least two runs first (see diagnosing hermaphrodites).
  • Narrowing to one male and one female every generation, then blaming "bad luck" for weak plants.
  • Publishing "F6 IBL" with no uniformity data.
  • Forgetting linkage drag: an unwanted trait close to your target gene can ride along for many backcrosses.

Evidence

  • Backcross genome recovery and linkage drag: Established [1].
  • Inbreeding depression from narrow populations: Established [2].
  • Trait-specific inheritance used above: Established / Supported [3][4].
  • DUS concepts: Established (regulatory standard) [5].
  • "Cubing" and "IBL" usage in cannabis: Grower practice terminology.

Questions this article answers

What is a backcross in cannabis breeding?

A backcross in cannabis breeding crosses an offspring back to one of its parents, called the recurrent parent, to recover that parent's genome while keeping a trait from the other parent, the donor. The first cross is donor x recurrent (F1); each following generation is a selected offspring crossed back to the recurrent parent again.

What does BX1, BX2, BX3 mean?

BX1, BX2 and BX3 count how many times offspring have been crossed back to the recurrent parent. On average a BX1 carries 75% of the recurrent parent's genome, a BX2 87.5% and a BX3 93.75%, so the donor share halves each time (25%, 12.5%, 6.25%). Real plants vary around these averages, and DNA near the target gene lingers longer.

What is cubing?

Cubing is grower jargon for backcrossing to the same clone three times, so a 'cubed' line is a BX3 with about 94% of that clone's genome on average. It recovers much of the clone's character, but the seeds still segregate and are inbred towards one individual. A cubed line is not a seed copy of the clone.

What is an IBL and how many generations does it take?

An IBL (inbred line) is a cannabis line inbred and selected over many generations until it breeds fairly true, but the term has no fixed definition or generation count. GenetixBay treats 'IBL' as a breeder claim unless backed by grow-out uniformity tables, tester rounds or genomic data showing low heterozygosity. An 'F6 IBL' label without data proves nothing.

How do I know if my line is stable?

A cannabis line is stable when grow-outs reproduce the target traits at a stated frequency, not when it reaches a certain generation. Count off-types in grow-outs of at least 50 plants; a suggested target is 5% or fewer off-types for key traits and 2% or fewer intersex plants, held for at least two generations and two environments.

How do I add one trait to my favourite strain?

To add one trait to a favourite cannabis line, cross it with a donor carrying the trait, then repeatedly backcross selected offspring to the favourite line. For a dominant trait, select carriers by eye; for a recessive trait such as autoflowering, add a self or sib-cross reveal generation or use a DNA marker. Prove the recurrent parent sex-stable first.

Sources

  1. Suza W, Lamkey K (eds). 2023. Quantitative Genetics for Plant Breeding. Iowa State University Digital Press. iastate.pressbooks.pub/quantitativegenetics/
  2. Charlesworth D, Willis JH. 2009. The genetics of inbreeding depression. Nature Reviews Genetics 10:783-796. doi.org/10.1038/nrg2664
  3. de Meijer EPM, Bagatta M, Carboni A, et al. 2003. The inheritance of chemical phenotype in Cannabis sativa L. Genetics 163(1):335-346. doi.org/10.1093/genetics/163.1.335
  4. Toth JA, Stack GM, Carlson CH, Smart LB. 2022. Identification and mapping of major-effect flowering time loci Autoflower1 and Early1 in Cannabis sativa L. Frontiers in Plant Science 13:991680. doi.org/10.3389/fpls.2022.991680
  5. UPOV. Test guidelines for hemp (Cannabis sativa L.), TG/276/1. upov.int/edocs/tgdocs/en/tg276_01.pdf
  6. Laverty KU, Stout JM, Sullivan MJ, et al. 2019. A physical and genetic map of Cannabis sativa identifies extensive rearrangements at the THC/CBD acid synthase loci. Genome Research 29(1):146-156. doi.org/10.1101/gr.242594.118

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F1, F2, F3 explained: what filial generations really mean

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How to document lineage evidence (GB-ID evidence levels L0-L4)

On GenetixBay every parentage statement is a claim with an evidence level: L0 unverified, L1 community reported with a source, L2 breeder reported, L3 documented by an accepted GB-DOC, L4 lab-confirmed by a genomic report consistent with the parent-child relationship. Breeders raise the level by attaching dated, original evidence and, where possible, by recording crosses on the platform as they happen.

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