"Selfing" is a term used in genetics and plant breeding to refer to the process by which a plant self-pollinates. This means that a single plant serves as both the male (pollen donor) and female (pollen recipient) in the reproductive process. In nature, selfing can occur in some species as a survival strategy to ensure reproduction when pollinators or other plants are scarce. In a breeding context, S1 stands for “first selfed generation”. An S1 seed is a product of self-pollination, where a female cannabis plant is induced to produce pollen and then pollinate itself or its clone.


Image: Cross-pollination and selfing by biology-forums.com

S1 seeds hold a distinctive place due to their unique genetic composition. This process leads to the production of seeds that have their genetics came solely from the mother plant’s genetics. They offer a level of genetic stability unattainable with other seed types, serving as a valuable resource for both commercial growers seeking uniformity and home cultivators interested in preserving specific plant traits.

S1 genetic explained

Homozygous vs heterozygous

Homozygous means that an organism has two identical alleles for a particular gene. For instance, if a pea plant is homozygous for the gene controlling seed color, it might have two alleles for green seeds (GG) or two alleles for yellow seeds (gg). In both cases, the plant is homozygous because both alleles are the same.

Heterozygous means that an organism has two different alleles for a particular gene. Using the same pea plant example, a heterozygous plant might have one allele for green seeds and one for yellow seeds (Gg). In this case, the plant is heterozygous because it has two different alleles.

In a heterozygous organism, the allele that gets expressed in the phenotype, or physical appearance, is the dominant one. In our example, if green is the dominant trait, a Gg plant will have green seeds because the green allele (G) is dominant over the yellow allele (g). 

What happens when the plant self-pollinates?

When the plant self-pollinates, each offspring gets a random mix of the parent's genes. 

If a parent plant is homozygous for a particular trait, it means that both alleles of the gene are the same. Therefore, S1 seeds will result in plants that are also homozygous like their parent plant. 

If a parent plant is heterozygous for a particular trait, it means that it carries two different versions (alleles) of a specific gene—one dominant and one recessive. When a heterozygous plant self-pollinates to create S1 seeds, the resulting offspring can express either the dominant or the recessive trait. Some S1 seeds (25%) will receive two copies of the dominant gene, 50% will receive one dominant and one recessive gene (still heterozygous like the parent), and others (25%) will receive two copies of the recessive gene.

The seeds that receive at least one copy of the dominant gene will express the dominant trait, while the seeds that receive two copies of the recessive gene will express the recessive trait. Overall, this means that even though S1 seeds are highly similar to the parent plant, there are still some variations of phenotypes due to how genes are combined.

Selfing pros

Preservation and stabilization of desired traits

If a cultivator has a particular cannabis plant with desirable traits—such as high THC or CBD levels, specific flavors, or resistance to certain pests or diseases— selfing provides a way to preserve these traits in future generations. In fact, selfing is the fastest way to stabilize a strain. It is also especially valuable in the preservation of unique or rare strains. 

S1 seeds are also feminized

S1 seeds are almost entirely feminized, unless you “self” a male plant. This means that they produce primarily female plants. 

Check to see if the wanted trait is dominant or recessive 

By cultivating a substantial number of S1 seeds, one can discern the dominant and recessive alleles within the plant's genetic profile. As an example, if you observe that roughly three-quarters of your S1 plants produce green buds, while only a quarter yield purple buds, it can be inferred that the green bud trait is the dominant characteristic, while the purple bud trait is recessive. Knowing this information can be extremely valuable when creating a new strain based on a desired characteristic.

Creating unique phenotype plants with recessive genes

Finally, recessive traits are often masked unless two copies of the recessive gene are present—one from each parent. A recessive trait will only be expressed if the organism is homozygous for that trait, meaning it carries two copies of the recessive gene.

S1 seeds, which are created by inducing a female plant to self-pollinate, can help bring out and stabilize recessive traits in cannabis cultivation. This is because self-pollination can result in offspring that carry two copies of the same allele for a particular trait when the parent is heterozygous.

Selfing cons

The Risks of Hermaphroditism

While S1 seeds offer many advantages, they also present certain challenges. One risk involves the potential for hermaphroditism. As S1 seeds are produced through stress-induced self-pollination, the subsequent generations may also exhibit a higher propensity to become hermaphrodite under stress, which could lead to unwanted pollination and seedy buds.

Possible Genetic Bottlenecks and Loss of Genetic Diversity

S1 seeds, being genetic clones of the parent plant, can result in a genetic bottleneck if used exclusively over multiple generations. The lack of genetic diversity might make the crops more susceptible to diseases and pests that the parent plant wasn't exposed to. It could also limit the development of new traits that could be beneficial in future plant breeding efforts.

Unintentionally stabilize negative, unwanted traits

While establishing a stable strain, breeders might unintentionally solidify less desirable characteristics. Over time, their cultivated strain might consistently exhibit certain unwanted features.

The Production Process of S1 Seeds

The Process of Stress-Inducing for Hermaphroditic Transformation

To produce S1 seeds, a female cannabis plant must be manipulated to generate male flowers, a process called hermaphroditic transformation. This is typically achieved by exposing the plant to certain stressors—like interrupted light cycles or the application of a colloidal silver solution. The stress induces the plant to produce male pollen sacs as a survival mechanism, enabling self-pollination.

Harvesting and Selection of S1 Seeds

Once the female plant has self-pollinated, it will produce seeds that carry its exact genetic information. These seeds are harvested once they reach full maturity, typically indicated by a darker color and hard shell. To maintain the high-quality genetic traits, growers selectively choose seeds that display optimal health and maturity.

Quality Assessment and Storage of S1 Seeds

Post-harvest, the seeds undergo a quality assessment to ensure they are free of molds, pests, and diseases. Proper storage is also vital to maintaining the viability and germination rate of the seeds. Typically, S1 seeds are stored in a cool, dry, and dark environment to prolong their lifespan and preserve their genetic quality. 

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