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JARS v59n4 – What’s the Difference Between F1 and F2?

What’s the Difference Between F1 and F2?
Donald W. Hyatt
McLean, Virginia

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.

JARS v59n4 – What’s the Difference Between F1 and F2? What’s the Difference Between F1 and F2? Donald W. Hyatt McLean, Virginia At the 2004 Banquet of the Potomac Valley Chapter ARS,

F1 Generation

F1 Generation Definition

The F1 generation refers to the first filial generation. Filial generations are the nomenclature given to subsequent sets of offspring from controlled or observed reproduction. The initial generation is given the letter “P” for parental generation. The first set of offspring from these parents is then known as the F1 generation. The F1 generation can reproduce to create the F2 generation, and so forth. Scientists use this designation to track groups of offspring as they observe the genetics of various generations.

Examples of F1 Generation

A Monohybrid Cross

When the “Father of Genetics”, Gregor Mendel, was first unfolding the secrets of pea genetics, he started by producing lines of pure-breeding peas. Peas are a variety of plant which can self-fertilize, meaning the male part of the plant can fertilize the eggs produced by the female part of the plant. When allowed to self-fertilize, these plants would produce offspring with the same traits. For example, the pea pods on one plant and all its offspring would produce green pods, while another plant would produce all yellow pods. To unlock the secrets of how these traits were passed to offspring, Mendel decided to cross these two lines of plants. Mendel took the pollen from yellow-pod plants and transferred it to green-pod plants. He then did the opposite cross, of green-pod pollen to yellow-pod flowers.

Scientist now designate these original two plants as the parental generation or simply the P generation. Once fertilized, the parental generation grows peas, which contain the genetic information for the first generation of offspring, or the F1 generation. Mendel planted these peas and noticed a curious fact about the color of the pea pods they produced: they were all green! The yellow-pod plants had contributed genetically to the F1 generation, but only green-pods were found.

Mendel had to do one further experiment to determine what was happening with the genetics controlling pod color. Mendel took a plant from the F1 generation, and allowed that plant to self-fertilize. He then planted and observed the offspring from this cross. Because it is a cross of the offspring, it represents the second filial generation, or F2 generation. Mendel observed that the F2 generation contained a mixture of green and yellow pods. Mendel showed that the 3:1 ratio of yellow-pod to green-pod plants could only be obtainable if both parents carried a copy of both the yellow and green alleles, and that the yellow allele had to be dominant over green.

Modern scientists now describe the cross of Mendel’s F1 generation as a monohybrid cross. The individuals in the cross all had one allele for green pods and one allele for yellow pods, making them hybrids. This cross only examined one trait, however many more traits can be observed at once.

A Test Cross

One problem Mendel ran into while breeding his peas is that in order to insure that he had a pure-breeding plant he had to breed the plant for several seasons to ensure it would only produce one variety of offspring. Knowing modern genetics, we can simplify this process. In contrast to the last example, the color of the peas INSIDE the pod works differently than the color of the pod itself. In fact, we know that the opposite is true: the yellow color allele for peas is dominant while the green color is recessive.

You pick up a handful of yellow seeds. How do you know which ones contain two dominant alleles (YY) and which ones are hybrids (Yy). The hybrids hide the green allele, which will be expressed if two green alleles find their way to the same organism. Where Mendel would self-fertilize each pea for many generations to purify out the hybrids, we can do it with one simple cross, known as a test cross. Look at the image below.

In a test cross, we take our unknown dominant seed, grow it into a plant, and fertilize it with a plant grown from a green seed. We know that green peas must contain two recessive alleles (yy). Therefore, one of two things can happen. We know that the yellow-pea plant has at least one dominant allele, but we don’t know what the other allele could be. The offspring of this cross, the F1 generation, can have two outcomes. Either the seeds will be all yellow, or they will be half yellow and half green. All yellow seeds in the F1 generation means that the unidentified seed we started with had two dominant alleles (YY). Only this could mask the green alleles present in the other parent. If the F1 generation produces a half and half mix, we know that the other allele in the parental yellow seed had to be a recessive allele, and that the parental yellow-pea plant is a hybrid.

1. Two pea plants are crossed. Both are homozygous for the genes controlling flower color. One produces purple flowers, while the other produces white flowers. What is the ratio of offspring in the F1 generation if the purple allele is dominant?
A. 1:1 Purple to White
B. All White
C. All Purple

2. You are a scientist studying a new species of fish. It is found that the fish come in two varieties, blue and red. Through other experiments, scientists have determined that red is dominant. You have a red fish, and you want to know if he is homozygous or heterozygous for the trait. What should you do?
A. A Test Cross
B. Breed with other red fish
C. Cross your fingers

3. A scientist is breeding daisies and studying their traits. He takes two plants to begin his experiments with. He collects their seeds, and grows the plants. He then crosses these plants with each other and collects the seeds they create. These seeds are again grown, crossed, and the seeds collected. This final round of seeds is planted and grows into plants. What generation do these plants represent?
A. F1 Generation
B. F5 Generation
C. F4 Generation

References

  • Hartwell, L. H., Hood, L., Goldberg, M. L., Reynolds, A. E., & Silver, L. M. (2011). Genetics: From Genes to Genomes. Boston: McGraw Hill.

The F1 generation refers to the first filial generation. Filial generations are the nomenclature given to subsequent sets of offspring from controlled or observed reproduction. The initial generation is given the letter “P” for parental generation.