Cannabis normally requires long dark periods to trigger flowering, but there are varieties that will flower regardless of the lighting conditions. These are known as “autoflowering” varieties. Not all cannabis traits are as simple to explain, which is part of what makes autoflowering a good example for beginning breeders to start with. For the purposes of this article, a few exceptions and special cases have been omitted for clarity, since even at the most basic level, it is a complicated enough topic that to some may seem confusing at first.

Gregor Mendel, a friar who conducted experiments with pea plants in the mid-1800s, tracked traits such as seed shape and flower color, and developed basic rules for understanding genetic inheritance such as the concept of dominant and recessive traits. Although some traits are complicated and require more than one gene to express, autoflowering is one of the most basic types of genetic traits and follows the laws of simple recessiveness and dominance. Specifically, it is a recessive trait.

Of importance to breeders, both experienced and aspiring, autoflowering is an easily observable simple Mendelian recessive trait. It is generally quick and easy to determine if a given adolescent plant is autoflowering or standard just by keeping it in short (or non-existent) dark periods for a few weeks. If the plant stays in growth, it is standard, if it flowers it is autoflowering.

Cannabis is a diploid, in other words, it gets one chromosome from the pollen of the father, and one from the ovum of the mother. This means it will have two genes (one from each chromosome strand), each of which can be one of two alleles, either autoflowering or not.

If we refer to the photodependant (standard) allele as P (upper case P), and the non-photodependant allele as p (lower case p), then the child plant will receive two—one from each parent. If the father is true breeding (homozygous) for photodependancy, it will have the allele P on both genes—the same is true for the mother. Seeds made from such pairings will result in offspring that is also true breeding for photodependancy. The father will contribute either a P or a P (because he has two big Ps, and will pass on one or the other). The mother will contribute either a P or a P (because she has two, and will pass on one or the other).

While technically there are four combinations possible, effectively it doesn’t make much difference because all of the combinations result in PP (true breeding for photodependancy). This is why breeding a standard cannabis plant with another standard cannabis plant will result in standard cannabis offspring. The same is true of autoflowering.

If both parents are true breeding for autoflowering, they will each have pp, and their offspring will autoflower (since they will receive a p from each parent). However, if one parent is PP (standard) and the other is autoflowering (pp), then all of the seeds will be Pp, since they will get one of the two P from the first parent, and one of the two p from the second parent. Having both the allele P and the allele p (heterogeneous for autoflowering) makes them a hybrid.

Since they have the alleles for both photodependancy and autoflowering, their phenotype (physical expression) will depend on dominance. Dominance determines which one breaks the ties in these instances. In this case, photodependancy is dominant, so the resulting Pp seeds will all be photodependant. This is why the first generation after crossing a true breeding standard plant with a autoflowering plant will create seeds that are photodependant and will not autoflower. However, the hybrid seeds aren’t useless. If the heterogeneous seeds from the above are crossed together, then the autoflowering trait will reappear:

  • The hybrid father will contribute either his P or p.
  • The hybrid mother will contribute either her P or p.
  • The resulting combinations are either PP, Pp, pP or pp.
  • The PP will be true breeding for photodependancy, and will show photodependancy.
  • The Pp and pP will be heterogeneous for photodependancy but will still show photodependancy because of dominance.
  • The pp will be true breeding for autoflowering, and will show autoflowering.

It is important to note that because of dominance there is no way to visually tell the difference between PP, pP and Pp. They will all be photodependant—only the recessive pp from this generation can be easily identified as true breeding.

A practical experiment can illustrate the above:

Step 1: Take a standard cannabis plant and cross it with an autoflowering variety. It doesn’t matter which supplies the pollen as long as one is male and the other female.

Step 2: Grow out the resulting seeds. They should act as if they were standard seeds. Select the best male and at least one female and cross them together.

Step 3: Grow out those seeds under growth (no long dark period) lighting. Due to the reasons explained above, there should be about 75 per cent that don’t autoflower and 25 per cent that do. The more seeds planted, the closer the results should be to the 75 to 25 per cent ratio.

One benefit to breeding for a recessive trait is that all of those that autoflower would have been true bred for autoflowering and can be bred together to make more autoflowering seeds.

Even if a breeder isn’t fond of autoflowering varieties, the process and learning experience of working with them can help expand their understanding of how genetic traits work in a way that is easy to see and will give tangible results and feedback. Applying Mendelian genetics to simple traits in the real world can not only help cement an understanding of the basic principles of genetics, but can impress onlookers when predictions are proven accurate, or at least reasonably close.