What's the Difference Between F1 and F2?
Donald W. Hyatt
At the 2004 Banquet of the Potomac Valley Chapter ARS, chapter member Gray Carter asked our speaker John Weagle what the terms F1 and F2 meant in hybridizing. I will try to explain those terms and suggest why many breeders use the technique to reach desired goals.
The term "F1" means the "first filial generation," or the initial cross between two genetically distinct plants. Often an F1 cross does not yield the desired goals because some traits do not show up in those first generation seedlings. For instance, what might one expect from a cross between an orange azalea species with a purple one? Purple color is dominant over orange in azaleas so all seedlings would likely be purple and not some ugly mix of those two shades. Every seedling does carry a gene for orange color but that trait is recessive and does not appear.
An "F2" cross is the next generation, or the result of crossing two sister seedlings from the F1 cross. Selfing an F1 plant produces an F2 also. Using the same example as before, if we crossed two of those purples from the F1 generation, the seedlings in the F2 cross often show the full range of possibilities, both purples and oranges.
Usually hybridizers want to combine the best traits from two different species when they make that initial F1 cross but don't reach their goals till the F2. We need to know how the genes work in order to understand why that happens. Let's look at an example using both color and height.
Unlike flower color, height is not typically a simple dominant and recessive trait. It is often an average of the two growth habits. So, what should we expect if we crossed a dwarf purple species with a tall orange in the quest for a dwarf orange?
The dwarf purple would have a gene for flower color that I will show as an upper case "C" since purple is dominant. I'll represent the gene for dwarf height with a lower case "h". The tall orange would have genes for each characteristic too, orange color represented by "c" since orange is recessive, and tall height with the gene "H".
Most normal organisms are "diploid," having two sets of genes for every characteristic. Species are often pure (having identical genes) in their genetic makeup (also called homozygous). Thus, the dwarf purple azalea would have two genes for each trait, two for purple (CC) and two for dwarf (hh), or the genetic makeup of CChh. The tall orange species would have a similar genetic makeup, two genes for orange color and two for tall height, or ccHH.
Since every seedling gets half of its genes from each parent, all plants in the F1 generation would get Ch from the dwarf purple and cH from the tall orange giving every seedling the same genetic makeup, CcHh. Those plants would be purple because of the dominant color factor but medium height since that trait is just an average.
In the next generation, or the F2 cross, the genes get reshuffled again so there are many possibilities. As before, half of the genes come from each parent but there are lots of choices now. The F2 results are listed in the chart below.
It turns out that three fourths of the plants will be purple. Some are pure (CC) like the original species but others carry both genes (Cc) just like the F1 parents. Only one fourth of the seedlings will have orange flowers since that happens when both recessive orange genes (cc) appear together.
We get a variety of heights now: dwarf, medium, and tall too. Approximately one sixteenth of the plants would reach our goal: orange color and dwarf height, or cchh. If we cross any two F2 plants, we would get an F3 but that gets complicated!
In reality, azalea flower color is controlled by many sets of genes so lots of unpredictable things can happen when hybridizing. The azalea cross John Weagle discussed was an F1 hybrid of a dwarf white form of R. kiusianum and a dwarf orange selection of R. nakaharae. A white crossed with an orange but the F1 plants were all purple! Why?
One possible explanation for the observed outcome was that the R. kiusianum carries purple genes. Maybe the white flowered form is white because it is unable to produce pigment of any kind, possibly controlled by a recessive gene. It was basically a purple azalea but just could not produce any color. When crossed with the orange R. nakaharae, the resulting F1 seedlings got those dominant purple genes from R. kiusianum but now the ability to produce color from the R. nakaharae parent. This could explain why all the F1 progeny were purple.
Now he moved to the F2 generation by crossing two F1 sister seedlings. The genes got reshuffled again. He saw purples and oranges as predicted by the prior example, but he saw a few whites too. The whites could happen if seedlings ended up with two recessive genes that inhibited color expression. However, he also got other shades like pink and red, which just means color inheritance is more complex than we imagine. If scientists ever map the full azalea genome, we might finally understand how everything works.
There is a humorous story related to the unpredictable results in inheritance. A lady once told George Bernard Shaw that he had the greatest brain in the world and she had the most beautiful body, so they ought to produce the most perfect child. He replied, "What if the child inherits my body and your brains?" He declined the offer.