Mendel’s Famous Pea Plant Experiment


By Robert Hazen, George Mason University

In the 19th century, the role of parents in genetics was becoming clear, but there was a major mystery: how does a fully formed plant or animal develop from that single, fertilized cell? You needed to have a new experimental approach, and that approach was provided by the Czechoslovakian monk Gregor Mendel, the founder of modern genetics.

Mendel's pea experiment
Gregor Mendel conducted experiments with pea plants based on which he formulated four laws of classical genetics. (Image: N.Vinoth Narasingam/Shutterstock)

From a Monk to a Scientist

Gregor Mendel, who lived from 1822 to 1884, grew up in a home with his parents and many close relatives being farmers, and this certainly influenced his future career in science. He entered the Augustinian monastery at Brno at the age of 21.

This particular monastery actively promoted study and learning, and it had an experimental research garden, of all things. Here, Mendel learned much about the experimental method, the scientific method, which he could apply to the study of genetics.

He engaged in experiments on sheep breeding and hybridization of plants. He was also allowed to study natural history. He was also sent to the University of Vienna.

Why Did Mendel Choose Peas?

Mendel’s famous genetic research on pea plants was conducted, between 1856 and 1863, in that monastery garden. He studied pea plants, and focused his attention on seven distinctive traits of pea plants. In particular, he saw that some pea plants were short, and some plants were tall; never in between. Some pea plants had white flowers, and others had purple flowers. Some had smooth pea pods; some had wrinkled pea pods.

Peas always displayed one trait or the other, and there were seven of these traits in total. This is unlike quite a number of organisms in which there are intermediate traits. For example, humans don’t come in just two varieties, short and tall; there are all different sizes in between. But these pea plants that Mendel was studying were either short or tall; a great advantage when you’re trying to study the origin of traits, and the passing of traits from one generation to the next.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

The Experiment

Mendel started by establishing populations of purebred plants. These are populations that always bred true: short plants that when cross-bred with each other always produced short plants; tall plants which, when crossed with others, always produced tall plants. He kept these populations separate to begin his experiments, so he had a whole series of populations that showed these different pure-bred traits.

picture of pea plants
Mendel’s choice of pea plants for the experiments allowed him to study the traits accurately since the pea plants had limited number of traits. (Image: Aleandro/Shutterstock)

Pea plants are normally self-pollinating. That is, the plant actually produces pollen, and accepts the pollen from itself, and new pea plants are then self-produced—a kind of cloning mechanism.

Mendel removed the male sex organs from the plants that he wanted to study, and he hand-pollinated these plants with pollen from a pre-selected father. So the male pollen was then carefully transferred from one plant to another.

Generations of Experiments

Mendel began his experiments by studying a single trait, for example, tallness. He would take a whole group of plants, and crossbreed a purebred tall plant with a purebred short plant. In the first generation of these experiments, every one of the hybrid plants of that first generation turned out to be tall. Mendel then called this trait, the one that appeared in the first generation, the dominant trait. He said that tall plants were dominant; short plants had the recessive trait.

It should be noted here that recessive and dominant traits do not mean “better” and “worse”. The idea of dominant and recessive only has to do with whether or not a trait is expressed, whether it appears in the organism.

That was the first generation, the hybrid generation of a tall and a crossed short; everything’s tall. Interestingly, when you crossbreed those hybrids with each other, you then find that 1/4 of the offspring, on average, are short, and 3/4 are tall. In the next generation, the short plants that are crossbred with each other only breed short plants; this is in the third generation, now. The tall plants, when crossbred, give you a combination of short and tall; you get a rather complex sequence of events.

More Findings

Mendel then did a whole series of experiments where he considered combinations of traits—two traits at once. For example, round versus wrinkled seeds, and yellow versus green seed color. What he found was that the round shape of a seed and the yellow color are the dominant traits, so wrinkled and green peas have the recessive traits; and he crossbred those.

What he would find is that when he had pure-bred round, yellow plants, and pure-bred wrinkled, green plants, when you cross-bred them, the first generation of hybrids were always round, yellow peas. In the next generation, you found a distribution that out of every 16 plants, on average 9 had round yellow seeds, 3 had round green seeds, 3 had wrinkled yellow seeds, and 1 out of 16 had the double-recessive trait of wrinkled green seeds. Mendel showed that this distribution is exactly what you’d expect if the two traits were completely independent of each other.

Mendel’s Laws of Genetics

From this vast accumulation of data, he derived four key ideas, which might be called the four laws of classical genetics.

  • First, there exist traits, what he called “atoms of inheritance”, what we now call genes.
  • The second law is that each individual carries two genes associated with every trait: one of them from the father, the other from the mother. Equal participation in the genetics of the offspring.
  • The third idea is that genes come in different forms, the different alleles. You have dominant alleles that are expressed; you have recessive alleles that are not expressed, unless both alleles are recessive.
  • And fourth, the idea that different alleles are sorted and distributed randomly, that all combinations of alleles are equally likely; although we now realize that in humans and in many other organisms this is not a general law.

Common Questions about Gregor Mendel and the Study of Genetics

Q: Who is called the founder of modern genetics?

Gregor Mendel is known as the founder of modern genetics for his experimental approach in genetics.

Q: When did Mendel conduct his famous pea plant experiment?

Mendel’s famous genetic research on pea plants was conducted between 1856 and 1863.

Q: Why were pea plants chosen for the experiment?

Peas always displayed one trait or the other, and there were seven of these traits in total. This turned out to be a great advantage for studying the origin of traits, and the passing of traits from one generation to the next.

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