# 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.

- Canonical: https://genetixbay.com/knowledge/breeding/backcrossing-ibl-stabilisation.html
- Category: Breeding · Level: Breeder
- Evidence: Established · Status: Draft: awaiting expert review
- Stage: Any stage · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and 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](https://genetixbay.com/knowledge/glossary/f1.md)); 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](https://genetixbay.com/knowledge/glossary/inbred-line.md)) 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](https://genetixbay.com/knowledge/glossary/heterozygous.md). 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](https://genetixbay.com/knowledge/glossary/dominant.md) trait, select carriers by eye; for a [recessive trait](https://genetixbay.com/knowledge/glossary/recessive.md) such as [autoflowering](https://genetixbay.com/knowledge/glossary/autoflower.md), add a self or sib-cross reveal generation or use a DNA marker. Prove the recurrent parent sex-stable first.

**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](https://genetixbay.com/knowledge/glossary/cbd.md), 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](https://genetixbay.com/knowledge/glossary/codominant.md) trait** ([chemotype](https://genetixbay.com/knowledge/glossary/chemotype.md) [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](https://genetixbay.com/knowledge/glossary/segregation.md) 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](https://genetixbay.com/knowledge/glossary/inbreeding-depression.md) [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](https://genetixbay.com/knowledge/glossary/evidence-levels-l0-l4.md) 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](https://genetixbay.com/knowledge/diagnosis/hermaphrodites.md)).
- 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.

## Sources

1. Suza W, Lamkey K (eds). 2023. Quantitative Genetics for Plant Breeding. Iowa State University Digital Press. https://iastate.pressbooks.pub/quantitativegenetics/
2. Charlesworth D, Willis JH. 2009. The genetics of inbreeding depression. Nature Reviews Genetics 10:783-796. https://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. https://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. https://doi.org/10.3389/fpls.2022.991680
5. UPOV. Test guidelines for hemp (Cannabis sativa L.), TG/276/1. https://www.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. https://doi.org/10.1101/gr.242594.118