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.

Test Cross

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

Answer to Question #1
C is correct. The F1 generation will be all purple here. The purple parents have the genotype (PP) if they are homozygous for the dominant allele. The white-flowered parents have the genotype (pp). Therefore, every offspring will receive one dominant allele and one recessive allele, or (Pp). Because P is completely dominant, the F1 generation can only produce purple flowers, but they all contain an allele for the recessive white flowers.

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

Answer to Question #2
A is correct. In this case, a test cross will reveal whether the fish is (RR) or (Rr). To do a test cross, you would breed this fish with a blue fish, known to be (rr) as blue is the recessive allele. If the F1 generation is red, the red parent has a (RR) genotype. If the F1 generation is half red and half blue, you know the original fish has the (Rr) genotype.

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

Answer to Question #3
C is correct. These plants represent the fourth filial generation, or the F4 generation. The plants that the scientist is breeding don’t have to be pure and their genetics don’t need to be know. The designation of generations is simply a device to allow the scientist to track offspring and the genes they carry.


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