GenetixBayEST. 2009
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 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.

What does BX1, BX2, BX3 mean?

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.

What is cubing?

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.

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

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.

How do I know if my line is stable?

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.

How do I add one trait to my favourite strain?

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.

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

Related

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

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S1s and selfing: what happens when a plant pollinates itself

An S1 is seed from a plant pollinated by its own pollen, usually by reversing one female clone or branch. S1s are feminised, and they are not copies of the parent: each S1 seed is a new genotype. Selfing halves heterozygosity per generation, reveals hidden recessives (good and bad), and is a fast way to test what a clone carries. Done carelessly, especially S1 on S1 from plants that herm, it is one of the main ways herm-prone lines are made; only self proven-stable females and cull every intersex offspring.

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Inbreeding depression and hybrid vigour

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Breeding autoflowers: getting the trait into a photoperiod line

Because autoflowering is recessive in mapped populations, a photo x auto F1 is all photoperiod. You recover autos in the F2 (about 1 in 4) or by backcrossing the F1 to the auto parent (about 1 in 2). To make an auto version of a photoperiod line, alternate crossing to the photo parent with a generation that reveals autos, or use a DNA marker to keep carriers.

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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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