Mendel's Pea Plant Experiment

Updated: Nov 8, 2020

Peas are arguably one of the most amazing vegetables to exist on the planet. No, not because of its taste, but rather because of its contribution to science. Peas - the green circular-shaped beans that we eat - are actually a really important component of science! These peas contribute to our current knowledge of genetics and how traits are passed down. They also provide crucial knowledge on why you inherit your parents’ traits. Wondering how? Well, PEAs (please) read on to find out!


Gregor Mendel

Before we dive into the experiment itself, we need to learn some basic information about the scientist who carried it out: Gregor Mendel. He is the scientist that performed the pea experiment, and is often known as the “father of genetics.” Surprisingly, this title mainly came from his pea plant experiments in his backyard.


The Pea Plant Experiment

General Information

Mendel studied the inheritance (passing of genetic information) of peas using different characteristics: pea height, flower color, and other traits. Note that all these traits are traits that are controlled by only one gene, and that each of these traits only have two distinctive forms. In other words, these traits are controlled by only two alleles (one from each parent), and there are only two possible outcomes from the experiment. For his experiment, Mendel made sure that the pea plants he used were only able to produced purple and white flowers.


Why Peas?

You might be wondering, out of all the plants in the world, why did Mendel decide to use pea plants as the model for his experiment? Well, there are lots of reasons:

  1. They have lots of traits that only have two possible phenotypes (the physical characteristics of an organism. For example, your eye color and the flower height are phenotypes), so it was easy for Mendel to observe the results and, and also prevented him from having to make any assumptions.

  2. They have fast life cycles, and can produce lots of seeds at once.

  3. They are easy to cross pollinate: all he had to do was transfer the pollen of one plant to another.


Self Pollination VS. Cross Pollination

Mendel performed these experiments using cross pollination. Usually, plants self pollinate, meaning that they pollinate themselves and create offspring without another “parent” plant. After many generations, these plants will be true-breeding, meaning that their offspring will carry the exact same genetic information and traits as their “parent.”. Cross pollination, on the other hand, involves producing offspring through two different parents. Therefore, their offspring will have genetic information from both parents.


Mendel was able to cross pollinate pea plants with different traits (Ex: crossing a tall plant with short plant) and observe the characteristics of their offspring.


P, F1, and F2 Generations

Another important thing to understand about the experiment is the different generations of the pea plants.


The P generation, or parent generation, are the plants that were produced from self pollination. They are the start of the experiment, and are true bred.


The F1 generation, or first filial generation, is the first generation of hybrids; they are produced from cross pollination in the P generation.


The F2 generation, or second filial generation, resulted from the F1 generation’s self pollination so they can observe how genetics are passed on (more information in the Punnett Square Section).


Dominant Vs. Recessive Traits / Alleles

There are dominant and recessive traits for each characteristic. Similarly, there are dominant and recessive alleles. Dominant refers to the trait that will always be present as long as there is at least one dominant allele ,while recessive gets covered up by the dominant allele. Usually, a dominant allele is represented using a capitalized letter while a recessive allele is represented by a lowercase letter.


Results




Note: the yellow circles represent the dominant trait, while the green circles represent the recessive trait. In the P generation, Mendel cross pollinated two plants: one with two dominant alleles (DD) and another with two recessive alleles (dd).


Note: In this example we will use D and d to represent the alleles. However, whichever letter you choose does not matter.


The result of that crossing was four plants with dominant traits. However, all of them actually have one dominant allele and one recessive allele.


Afterwards, the F1 generation self-pollinated, which basically meant that the two genotypes (the genetic makeup of an organism) would be Dd and Dd. The F2 generation’s result is three dominant and one recessive. You might be wondering, if all of the plants in the F1 generation are dominant, how come there is one recessive plant in the F2 generation?


Punnett Squares

The diagram above can also be represented using Punnett Squares. Punnett Squares are diagrams that can be used to predict the results of a cross. They are useful when it comes to analyzing the genotype and phenotype of an organism. We will be constructing a Punnett Square to predict the F1 generation cross that produces the F2 offsprings. Constructing a Punnett takes a few easy steps:

  1. Draw a 2x2 square and write the genotypes of each parent (or if it’s self pollination then write the parent’s genotype twice) on the upper and right side of the 2x2 square. So, in this case, we would put Dd and Dd.

  2. Put both the allele of the row and column into the grid. For example, the resulting genotype of the center grid (the one that says DD, since both the row and column of that grid is D, would be DD.

As shown in this Punnett Square, since both of the alleles of the parent (or in this case, both the parents are the same plant) have a recessive allele, there is a 25% chance in which the offspring will inherit two recessive alleles: one from each parent. So, this means that there is still a chance of the offspring being recessive even if the parent is dominant.


Findings of His Experiment

By implementing cross pollination, Mendel made some observations that impacted our understanding of genetics. The main results of this experiment includes:

  1. Law of Independent Assortment: All of the traits are inherited independently. This means that the plant height has two separate alleles and the color has two other alleles. They do not affect each other’s inheritance.

  2. Law of Segregation: Since each trait is controlled by two alleles, during cross pollination, the two alleles of each parent-plant separates from each other, and the child plant gets one allele from each parent.


Not All Traits are Mendelian Traits

Unfortunately, genetics is so much more complicated than what Mendel thought it was. The traits that fit his theory are known to be Mendelian traits. These are the traits that are only controlled by one gene, have only two allele possibilities, and that have to have one possible allele be completely dominant over the other. However, there are countless traits that do not follow this rule: some traits are controlled by more than two alleles, while some are controlled by more than one gene, and there are still many other possibilities. For example, intelligence isn’t controlled by only ONE gene, but rather, by a lot of genes:therefore we cannot use Punnett Squares to determine how “smart” someone is simply based on their parents’ alleles.


Conclusion

Wow, who would’ve thought peas are this important? Well, next time you eat peas, just remember that they are an important part of a scientific discovery!







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