Why is the F2 generation “wilder” than F1 or F3???
Many times, growers will say, “I love digging through F2s” or “I’ll try to find a male from the F2.” The realization that there is something peculiar about F2 seeds may seem puzzling. It rubs against the assumption that as a line is worked through subsequent generations, there should be a stabilizing force that funnels phenotypic variance with each new generation. Yet, we can see a marked difference in expressions during the F2 vs. F1 or F3. This concept was realized by Gregor Mendel during his experiments with peas.
While studying inheritance, Mendel bred one pea that was true-breeding for smooth peas (SS) with a wrinkled pea (ss) and observed that all of the resulting progeny in the F1 generation were smooth peas.
This is not shocking to us now that we understand the mechanics of inheritance. The dominant “smooth” trait ensures that all of the F1 phenotypes are smooth peas (Ss). It is worth noting that while all of our F1 peas have a smooth phenotype, their genotype now includes the recessive wrinkled allele (s).
When Mendel selected 2 peas from the F1 generation for breeding, the F2 generation produced 25% wrinkled peas.
Let’s take a look at a similar example using cannabis. Assume a breeder was working on plant height. This breeder wanted tall plants so they used their true-breeding for tall height Haze (SS) with a new short Afghan Skunk (ss) that had another trait of interest.
To the breeder’s satisfaction, the F1 generation produces all tall offspring (Ss). Our breeder would like to further refine the line to shore up this particular trait along with a few new traits acquired and selects two plants from the F1 to create F2.
Now, the breeder is surprised to see 25% of the F2 plants sporting a short height (ss). While the F1 phenos were very uniform, the F2 generation seems to be throwing a curveball. Our breeder culls the 25% shorties and selects from the remaining tall plants for the F3 generation.
The F3 offspring now assume the uniform tall height seen from the F1. The added benefit to the F3 generation is that we now have 50% true-breeding tall plants (SS). This would mean that if the breeder selects 2 (SS) plants for F4, the resulting genotypes would all be true-breeding for tall height.
This example is a good illustration of the difference in goals between growers-for-smoke(GFS) and breeders. Chuckers and GFS will be most interested in phenohunting populations with no explicit interest in the genotype. A breeder is a genohunter, testing and observing populations to create desirable genetic arrangements.
How traits from Grandparent plants can reemerge in the F2
The F2 generation can see the reappearance of traits from grandparent plants that were considered “gone”. To illustrate how this can happen, let’s see what our hypothetical breeder is up to now.
Schwaggy Z has a great female plant that he would like to use for a project (called Project X). He buys a pack of Black Lights (Black Domina x NL#1) to find a suitable male for his project. Schwaggy has never seen the individual Black Domina or NL#1 that went into this cross, so he’s just hoping to find a nice indica male.
After popping the entire pack, he observes all of the plants are green with no variegation. He selects a male plant from this parent stock (P) and mates him to his great female plant that is also green with no variegation. All of the F1 plants are green without variegation and our breeder is feeling pretty good about results so far. Schwaggy continues with selection from the F1 and breeds to the F2.
To his surprise, the F2 offspring now have green leaves with and without variegation as well as black leaves with and without variegation. Schwaggy is at a loss to explain where these traits came from and worries about some kind of pollen contamination. How can this be?
The breeder of the Black Lights used a Black Domina with black leaves that have variegation and a Northern Lights #1 with green leaves and no variegation. We’ll use:
G – dominant Green leaf
g – recessive black leaf
N – dominant NO variegation
n – recessive variegation
The Black Domina’s genotype is double recessive for both traits (ggnn) while the NL#1 is (GGNN). Let’s see how these grandparents breed to make the seeds Schwaggy popped to find a male.
Now we can see that the black variegation was “washed out” in the creation of the pack of seeds Schwaggy bought to find a male for Project X. Since he never saw the grandparents and only saw all green/no variegation plants, he never suspected the black/variegation traits were lurking.
Schwaggy selects a male from this population (the pack of Black Lights) with the genotype (GgNn) to mate with his green/no variegated female (GGNN) to create F1 offspring.
All of the F1 offspring are green with no variegation, but the genotypes are now more diverse than the parent generation. Schwaggy selects a (GgNn) female and (GgNn) male to create F2 seeds.
To his shock, the F2 plants now produce:
~55% green leaves without variegation
~20% green variegated leaves
~20% black leaves without variegation
~5% black variegated leaves
The F2 generation saw the reappearance of a trait that was only seen from a grandparent plant and never manifested through the P or F1 generations.
With the aid of Punnett Squares, we are able to make sense of the F2 mystery.