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

> An F1 is the first generation from two parents; it is uniform only if both parents were true-breeding (inbred). An F2 comes from crossing F1 siblings and shows the most variation. Each later generation of sib-mating with selection increases uniformity slowly. In cannabis, most 'F1 hybrids' are crosses of heterozygous clones, so they behave like segregating F2s.

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- Category: Breeding · Level: Intermediate
- 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 does F1 mean on a seed pack?** F1 on a cannabis seed pack means the first generation from crossing parent A with parent B. It says nothing about uniformity unless both parents were inbred. Most cannabis F1s are crosses of two [heterozygous](https://genetixbay.com/knowledge/glossary/heterozygous.md) clones, which segregate like an F2 at many genes, so one pack can show several distinct [phenotypes](https://genetixbay.com/knowledge/glossary/phenotype.md).

**Why are my F1 plants not uniform?** Cannabis F1 plants are usually not uniform because the parents were heterozygous clones, not [inbred lines](https://genetixbay.com/knowledge/glossary/inbred-line.md). Uniform F1 hybrids, as in maize, come only from two [homozygous](https://genetixbay.com/knowledge/glossary/homozygous.md) parents that each pass on identical gametes. Elite cannabis clones are highly heterozygous [polyhybrids](https://genetixbay.com/knowledge/glossary/polyhybrid.md), so each makes many different gametes and the F1 varies from the first generation.

**Why are F2 seeds so variable?** F2 cannabis seeds are variable because every gene that differed between the original parents segregates 1:2:1 in the F2, so variation is at its maximum. With just 10 differing genes there are 3 to the power of 10, or 59,049, genotype combinations. That makes the F2 the worst generation for uniformity but the best for finding new combinations.

**How many generations to make a stable strain?** Making a stable cannabis strain usually takes many generations, and the count alone proves nothing. With full-sib mating, heterozygosity falls by only about 19% per generation, leaving about 41% after four generations and 27% after six; selfing halves it each time. Judge stability by measured uniformity in grow-outs, and avoid bottlenecking to one pair each generation.

**Is an F2 the same as an S1?** No, an F2 is not the same as an S1. An F2 crosses two F1 siblings, while an S1 is a single plant pollinated by itself, usually after reversal. Selfing halves heterozygosity in one generation, so an S1 reveals [recessive traits](https://genetixbay.com/knowledge/glossary/recessive.md) faster and is more inbred, whereas a sib-mated F2 keeps more diversity and maximum segregation.

**Short answer.** F1 = parent A x parent B. F2 = F1 x F1 (siblings, in cannabis). F3 = F2 x F2, and so on. The textbook promise ("F1s are uniform") holds only when both parents are inbred. Cross two heterozygous clones, which is what most cannabis "F1s" are, and you get variation from the first generation.

## What's happening

### The textbook case (inbred parents)
- P1 is AA BB CC..., P2 is aa bb cc... (both homozygous).
- **F1** is Aa Bb Cc... at every differing gene: genetically identical siblings, uniform, often vigorous ([hybrid vigour](https://genetixbay.com/knowledge/glossary/heterosis.md)) [2].
- **F2**: each gene segregates 1:2:1, so variation is maximal. With 10 differing genes there are 3^10 = 59,049 genotype combinations [1].
- **F3 and later**: with sibling mating and selection, heterozygosity falls and families become more uniform.

### The cannabis case (heterozygous parents)
Most elite cannabis clones are highly heterozygous polyhybrids [5]. Crossing two of them is genetically like an F2 at many genes: the "F1" segregates. That's why one pack of "F1 hybrid" seed can show several distinct phenotypes. Calling it F1 is correct by definition (first generation of that cross) but misleading about uniformity.

## How fast does heterozygosity fall?

| Mating system | Heterozygosity left after each generation | After 4 generations | After 6 generations |
|---|---|---|---|
| Selfing (S1, S2...) | x ½ | ~6% | ~1.6% |
| Full-sib mating | H(t) = ½H(t-1) + ¼H(t-2); tends to x 0.81 per generation | ~41% | ~27% |
| Random mating in a large population | ~no change | | |

Values are single-locus expectations starting from unrelated parents [2]; selection and linkage change them in practice. The point: sib-mated cannabis lines take many generations to approach [true breeding](https://genetixbay.com/knowledge/glossary/true-breeding.md), and "F5 [IBL](https://genetixbay.com/knowledge/glossary/ibl.md)" says more about effort than about measured uniformity.

## Worked example: chemotype in F1 and F2

THC-dominant inbred (B_T/B_T) x CBD-dominant inbred (B_D/B_D) [3]:

- F1: all B_T/B_D, all mixed type II. Uniform for chemotype.
- F2: 1 type I : 2 type II : 1 type III.
- F3 from a type III F2 pair: all type III. Fixed for that trait.

Note that being fixed for chemotype says nothing about the other few thousand genes.

## Naming conventions

| Label | Meaning | Uniformity to expect |
|---|---|---|
| F1 (inbred parents) | Uniform hybrid | High |
| F1 (clone x clone) | First cross of two heterozygous plants | Low to moderate |
| F2 | F1 sibling cross | Lowest; best for finding new combinations |
| F3-F6 | Further sib generations with selection | Rising |
| BX1, BX2 | Backcrosses to one parent | Moving towards the recurrent parent |
| S1 | Selfed | Variable; reveals recessives |
| IBL | "Inbred line", undefined generation count | Claim; check data |

## Step by step: making F2s that mean something

1. Grow out at least 10-20 F1s (both sexes for regular seed).
2. Select the best female and the best 1-3 males (by [progeny tests](https://genetixbay.com/knowledge/glossary/progeny-test.md) if you can).
3. Make the F2 cross and record parents as GB-IND IDs, not just the line name.
4. Grow a large F2 (see [pheno hunting](https://genetixbay.com/knowledge/breeding/selection-pheno-hunting.md) for numbers) and select.
5. Advance by crossing selected F2 siblings; to stabilise, keep the population from bottlenecking to a single pair every generation ([inbreeding depression](https://genetixbay.com/knowledge/glossary/inbreeding-depression.md)) [4].

## Common mistakes

- Expecting uniform F1s from two [clone-only](https://genetixbay.com/knowledge/glossary/clone-only.md) parents.
- Selling F2s as if they were stable. They are the opposite.
- Making every generation from one male and one female. That is close to selfing in its inbreeding effect, without the planning.

## Evidence

- Segregation, F1 uniformity from inbred parents, F2 variability: *Established* [1][2].
- Heterozygosity of commercial cannabis: *Supported* [5].
- Chemotype example: *Established* [3].
- Inbreeding effects of narrow sib-mating: *Established* in general plant genetics [4].

## Sources

1. Miko I. 2008. Gregor Mendel and the principles of inheritance. Nature Education 1(1):134. https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/
2. Suza W, Lamkey K (eds). 2023. Quantitative Genetics for Plant Breeding. Iowa State University Digital Press. https://iastate.pressbooks.pub/quantitativegenetics/
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. Charlesworth D, Willis JH. 2009. The genetics of inbreeding depression. Nature Reviews Genetics 10:783-796. https://doi.org/10.1038/nrg2664
5. Lynch RC, Vergara D, Tittes S, et al. 2016. Genomic and chemical diversity in Cannabis. Critical Reviews in Plant Sciences 35(5-6):349-363. https://doi.org/10.1080/07352689.2016.1265363