Every code on a seed packet — F1, F2, S1, BX3, “stabilised” — describes a specific breeding operation with a specific consequence for your grow room, and almost none of them are used correctly by the people printing them.
1. Genotype, Phenotype, and What a Label Promises
A seed listing is a promise about variation: how similar your ten plants will be to each other, and to the photograph. Generation codes are the only part of a listing carrying real information about that, because they say how many rounds of selection stand between your seed and the original cross.
Genotype is the gene variants a plant carries; phenotype is what you observe — height, aroma, resin, flowering time. Phenotype is genotype filtered through environment, so phenotype hunting only works if every plant gets identical treatment.
Cannabis is diploid: two alleles at each locus, one per parent. Two identical alleles is homozygous, two different is heterozygous.
2. Dominant, Recessive and Codominant Alleles
| Relationship | The heterozygote looks like | Practical effect |
|---|---|---|
| Dominant / recessive | The dominant parent | Trait vanishes a generation, then returns |
| Codominant | Both alleles at once | Nothing hides; phenotype reveals genotype |
| Additive or polygenic | Intermediate, continuous | Many loci, no clean ratios, slow to fix |
Recessive alleles ambush growers. A 2022 study in Frontiers in Plant Science by Toth and colleagues mapped the autoflowering trait in Cannabis sativa to a single locus they named Autoflower1, a recessive allele at one locus. In their segregating populations of 88 plants, 31.8% and 27.3% flowered under long days — both close to the 25% one recessive locus predicts. So crossing an autoflower with a photoperiod plant gives no autoflowers at all in the first generation, and a quarter of them back in the second.
3. Chemotype Inheritance: The Best-Documented Single Gene System in Cannabis
Most cannabis traits are messy. One is not. De Meijer and colleagues, in Genetics in 2003, proposed that the ratio between the two major cannabinoids is set by a single locus, which they called B, carrying two codominant alleles. One codes for a synthase converting the shared precursor cannabigerolic acid toward cannabidiolic acid, the other toward tetrahydrocannabinolic acid. Breeder shorthand: BD and BT.
| Chemotype | Genotype | Chemical profile |
|---|---|---|
| One | BT / BT | Tetrahydrocannabinol-dominant |
| Two | BT / BD | Both, roughly balanced |
| Three | BD / BD | Cannabidiol-dominant |
A worked Punnett square
Write one parent’s gametes across the top, the other’s down the side, fill in the combinations. Here, two balanced plants crossed — the classic way a balanced line goes wrong:
| BT | BD | |
|---|---|---|
| BT | BT/BT | BT/BD |
| BD | BT/BD | BD/BD |
De Meijer’s group observed exactly this 1:2:1 ratio in their own second-generation populations: a quarter revert to one chemotype, a quarter to the other, only half stay balanced. A balanced-cannabinoid plant cannot breed true by definition, because its defining genotype is heterozygous. “Stabilised balanced” seed either segregates, or comes from two homozygous parents — and then only the first generation is balanced.
The exceptions are real
De Meijer’s own paper noted the ratio within a chemotype was progeny specific: the class inherits cleanly, the exact number does not. Van Bakel and colleagues, in Genome Research in 2018, mapped these synthase genes physically and found them inside repeat-rich regions over 250 kilobases across, highly non-homologous between drug-type and hemp-type chromosomes, with extensive structural rearrangement. Strongly heritable and roughly predictable, then — not guaranteed.
4. Sex Determination: The X and Y System
Cannabis is normally dioecious, and sex is chromosomal. A 2014 molecular cytogenetics study in PLOS One characterised the system directly, confirming heteromorphic sex chromosomes: females homogametic, males carrying a distinct, larger Y chromosome.
| Cross | Pollen contributes | Expected progeny |
|---|---|---|
| Female (XX) × male (XY) | X or Y | About half female, half male |
| Female (XX) × reversed female (XX) | X only | All female |
The second row is the entire basis of feminised seed. But sex expression is not the same as sex chromosomes: stress, hormone treatment and light disruption all push genetically female plants to make pollen.
5. What a First Filial Generation Actually Is
F1 means first filial generation. In formal plant breeding it carries a stronger requirement: both parents must be homozygous inbred lines, distinct from each other. Only then does every seed share an identical genotype, which is what makes the crop uniform.
| Parents | Correct label | Seedlings uniform |
|---|---|---|
| Two distinct inbred lines | True F1 | Yes, genetically identical |
| Two unrelated non-inbred plants | First cross, loosely called F1 | No |
| Two siblings from one batch | Second filial generation | No |
| One plant selfed | S1 | Partly |
6. Hybrid Vigour: What the Cannabis Evidence Shows
Heterosis is the observation that a cross between two inbred lines often outperforms both parents. Honesty first: the overwhelming classic evidence comes from maize, after a century of hybrid breeding. Cannabis-specific evidence is much thinner, and a lot of what circulates in grower forums is maize genetics borrowed without acknowledgement.
Cannabis evidence does now exist. McDonald and Lubell, in HortScience in 2024, compared F1 hybrid lines against selfed inbred lines and reported the hybrids showed greater uniformity, greater plant dry weight and greater flower dry weight. A proof-of-concept study by Garcia-de Heer and colleagues in Horticulture Research used single-seed descent through up to six rounds of self-fertilisation across sixteen lines, then crossed the resulting inbred lines: the F1 hybrids were statistically more uniform than the inbred or original lines, seed yield rose between 3.9% and 155% above the mid-parent value, and all F1 plants began flowering within a 4-day window against 21 to 23 days for the parent material.
The reliable benefit of a true F1 is synchrony. Yield gain was real but wildly variable.
7. Why Most Seeds Sold as F1 Are Not True First Crosses
Making a true F1 means producing two homozygous parent lines first, then maintaining both indefinitely. The single-seed descent work above took roughly two and a half years to reach the sixth selfed generation, and only five of sixteen lines survived. Almost nobody in the seed trade does this; what is sold as F1 is usually a cross between two heterozygous plants, often themselves complex hybrids.
“F1 of two elite clones” is the clearest tell of all: clones are heterozygous by definition, so their offspring cannot be uniform. One question separates real from decorative usage: were both parents inbred, and to what generation? A breeder doing real work answers immediately.
8. The Second Generation: Where Everything Splits Apart
F2 is the offspring of two F1 siblings — where hidden variation is dealt out in new combinations. One locus gives 1:2:1 genotypes, two independent loci give 9:3:3:1 phenotypes under simple dominance, dozens give a spread. An F2 is an opportunity, not a finished product: the photograph shows one plant the breeder found in their own F2, and your odds of finding it again are below.
| Loci involved | Genotypes possible | Chance a seedling is homozygous for all target alleles |
|---|---|---|
| 1 | 3 | 1 in 4 |
| 2 | 9 | 1 in 16 |
| 3 | 27 | 1 in 64 |
| 4 | 81 | 1 in 256 |
9. Third to Fifth Generation: The Slow Road to Stabilisation
From the F2 onward, the breeder selects plants closest to the target and crosses them together. Each round of inbreeding roughly halves the proportion of loci still heterozygous.
| Generation | Expected heterozygosity under selfing | What you see |
|---|---|---|
| F1 | 100% | Uniform but genetically mixed |
| F2 | 50% | Maximum visible variation |
| F3 | 25% | Two or three recognisable types |
| F4 | 12.5% | One dominant type, some outliers |
| F5 | 6.25% | Mostly uniform |
| F6 and beyond | Under 4% | Commercially uniform |
The numbers apply only to traits under selection — neglect vigour and you lose it — and halving heterozygosity is not the same as reaching the target. As a rule of thumb, not a measured constant: F4 is the first generation worth planning a production run around, F5 or beyond genuinely predictable.
10. Inbred Lines and What Stability Really Means
An inbred line is a population inbred until it breeds true: cross two members, the offspring resemble the parents. The trade-off is that the inbreeding which makes a line predictable tends to reduce vigour — exactly why the F1 strategy exists.
A finished line has few hidden recessive alleles left and works well as a breeding parent — the opposite of a polyhybrid on both counts.
11. Selfed Seeds: How S1 Is Made and What It Costs
S1 is the first selfed generation: one female plant pollinated with its own pollen, after treatment reverses part of it to male flowering. No Y chromosome exists in the cross, so the seed is female. But an S1 is not a clone in seed form — the parent was heterozygous at many loci, and selfing deals those out much as an F2 would.
| Expectation about S1 seed | Accurate |
|---|---|
| All seedlings female | Yes, barring later stress-induced reversal |
| All seedlings identical to the mother | No |
| Recessive traits may surface | Yes, commonly |
| Genetic diversity collapses in one generation | Not necessarily |
Punja and Holmes, in Frontiers in Plant Science in 2020, examined seed formed on hermaphroditic inflorescences and found the seedlings exclusively genetically female, while cross-fertilised seed gave roughly a 1:1 sex ratio. They also found no measurable difference in genetic variation between selfed and cross-fertilised populations after one generation — a correction to the claim that one selfing round bottlenecks a line.
The hermaphrodite risk, stated precisely
Hermaphroditism is not a simple inherited trait; evidence points to it being largely stress-induced. The same 2020 work noted spontaneous appearance in roughly 5% to 10% of plants under commercial conditions, and prior research showed triggering by gibberellic acid, silver compounds and environmental stress. The risk is selection — a plant that reverses easily under treatment has, by definition, a low threshold for reversal, and selfing passes that threshold on. So never breed from a plant that threw pollen untreated.
12. Backcrossing: Recovering a Cutting-Only Mother in Seed Form
A backcross solves one problem: an exceptional plant exists only as a cutting and you want seeds carrying it. Cross it to something, then cross the offspring back to the original — the recurrent parent — repeatedly.
| Step | Notation | Theoretical genome share from the recurrent parent |
|---|---|---|
| First cross | F1 | 50% |
| First backcross | BX or BX1 | 75% |
| Second backcross | BX2 | 87.5% |
| Third backcross | BX3 | 93.75% |
| Fourth backcross | BX4 | 96.875% |
Those percentages are genome-wide averages, not guarantees for any one seedling. Critically, a heterozygous recurrent parent can never be perfectly recovered, because a heterozygous genotype cannot be fixed. BX3 seed of a famous cutting is a population resembling it closely on average, not a set of copies.
13. Polyhybrids and the Collapse of Meaningful Labels
A polyhybrid is a hybrid whose parents were themselves hybrids, several generations deep, with no inbreeding in between. Almost every modern cultivar with a compound name is one, and the name therefore stops predicting anything: two breeders can make the same-named cross from different parents and get populations sharing almost nothing.
Watts and colleagues, in Nature Plants in 2021, genotyped cannabis samples at over 116,000 genetic markers alongside chemical analysis — 297 samples chemically, 137 with both datasets — and found genome-wide genetic structure accounted for only about 37% of the variance in the Sativa and Indica labels applied to them. Their conclusion was direct: those labels do not reliably reflect genetic relatedness. What they partially tracked was terpene chemistry, myrcene correlating with Indica labelling though weakly, mapping to terpene synthase gene arrays.
So a polyhybrid name is a moderate guide to the breeder’s intended lineage, a weak guide to aroma family, and no guide at all to uniformity or to relatedness with another plant carrying the same name.
14. How Many Plants and How Many Generations You Need
Population size decides whether the genotype you want is in the room at all. The figures below are probability, not measured results, and assume independent loci with clean dominance — a best case.
| Independent loci to fix | Chance per seedling | Seedlings for about a 95% chance of at least one |
|---|---|---|
| 1 | 1 in 4 | 11 |
| 2 | 1 in 16 | 47 |
| 3 | 1 in 64 | 190 |
| 4 | 1 in 256 | 765 |
Generation count decides whether the trait stays put. The single-seed descent work above reached the sixth selfed generation in roughly two and a half years indoors; outdoors at one generation per season the same project is six years. As planning rules of thumb, not measured constants: 10 to 30 plants and one generation to find a keeper cutting; 50 to 200 plants over four to six generations for one simple trait; 200 or more over six-plus generations for a whole plant type.
15. Reading a Seed Listing Line by Line
| Listing text | Translation | Expect in the room |
|---|---|---|
| “First cross, untested” | Two parents crossed once | High variation, no data |
| “F2, pheno hunt recommended” | Honest segregating population | Wide spread; grow numbers |
| “F5, stable” | Several rounds of selection | Mostly uniform; verify with a test run |
| “Inbred line, landrace-derived” | Breeds true | Predictable, possibly lower vigour |
| “S1 of a named cutting” | Selfed | All female, variable, watch for reversal |
| “BX3 of a named cutting” | Three backcrosses | Close to the original on average |
| “Autoflower F1” | Likely a first cross | Check all plants actually autoflower |
That last row is a test anyone can run: because the autoflowering trait is recessive at a single locus, a properly made autoflowering batch should give essentially all autoflowering plants. If a meaningful fraction waits for a light cycle change, the batch was segregating and mislabelled. Finally, germinate a small test batch before committing a full room — six plants tell you more about uniformity than any paragraph, at the cost of one cycle instead of a year.
Now the part that matters more than the article. Members here breed in conditions no published study covers — tropical outdoor seasons, monsoon humidity, latitudes where photoperiod behaviour differs from every temperate dataset. If you have grown out an F2 and counted your phenotypes, post the numbers. If a selfed line produced reversals at a rate you can quantify, share it with the treatment method and the conditions. Bring your seedling counts, your segregation ratios, your failures and your photographs to Breeder’s Lab.
This article is part of the Asiannabis Community educational series on cannabis genetics and breeding. Content is for educational purposes within jurisdictions where cannabis cultivation is legally permitted.