Gregor Mendel's Pea Plant Experiment

Valorie Delp
pea shoot

Gregor Mendel is considered the father of modern genetics. He was an Austrian monk who worked with pea plants to explain how children inherit features from their parents. His work became the foundation of how scientists understand heredity, and he is widely considered a pioneer in the field of genetics.

Pea Plants and Mendelian Genetics

In Mendel's famous pea plant experiments, he deliberately cross-pollinated pea plants with obviously different features to discover some important things about how offspring inherit traits from their parents.

The Experiments

Mendel measured seven specific characteristics of pea plants:

  1. Smooth or wrinkled ripe seed
  2. Yellow or green seed albumen
  3. Purple or white flower
  4. Inflated or constricted ripe pod
  5. Green or yellow unripe pods
  6. Axial or terminal position of flowers
  7. Tall or dwarf stem length

What He Discovered

Between 1856 and 1863 Mendel experimented on the Pisum sativum, or pea plant, species. His experiments led him to make three generalizations:

  1. Offspring acquire one hereditary factor from each parent. This is known as the law of segregation.
  2. Different traits have an equal opportunity of occurring together. This is known as the law of independent assortment, and today's scientists understand this to be largely inaccurate. Some genes are in fact, linked together and appear more often together.
  3. Offspring will inherit the dominant trait, and can only inherit the recessive trait if he inherits both recessive factors. This is known as the law of dominance.

Most scientists of his day rejected Mendel's work. It wasn't widely accepted until after he died. During his lifetime, most scientists believed offspring inherited traits by blending, that is offspring inherited an 'average' of the traits of the parents.

Demonstrating Mendelian Genetics

Mendel is said to have tested over 28,000 plants to come to his conclusion. While the scope of his project is probably not realistic for you to recreate, you can study genetics using plants.

Who's the Father?

brassica rapa

Who's the Father is an experiment in which students will experiment on plants to predict observable traits. You can recreate the experiment using Wisconsin Fast Plants® (Brassica rapa) - which are designed specifically so students can use them to study genetics. They also grow faster - a complete life cycle takes 28-30 days. This experiment will take approximately six weeks of daily observations to complete. It's best suited for older students in middle school or high school who are studying genetics.

Materials

Instructions

  1. Construct your lighting and watering systems first. Wisconsin Fast Plants® need continuous fluorescent light, and a continuous supply of fertilizer and water. You can either build homemade versions of these, or you can purchase pre-made kits through Carolina Biological. Both options are linked above in the materials list.
  2. Plant the seeds (you do not need to use all of them) according to the growing instructions. You'll want to start by planting the non-purple, yellow-green leaf seeds (this will be referred to as the first-generation offspring, or O1.) Also plant the non-purple stem, hairless seeds. (These seeds are the mother seeds, referred to as P1). Make sure you label which is which!
  3. In approximately four to seven days, your plants should grow. Observe the stem and leaf colors of both sets of plants and record your observations in your lab notebook. The best way to quantify your observations is to count the phenotypes (count the number of plants that have non-purple stems, the number of plants that have yellow-green leaves, etc.)
  4. Discard the mother plants, but maintain the offspring plants.
  5. Write a hypothesis as to how the offspring plants inherited their observable genetic traits. For example, if you observe that most of your offspring plants have non-purple stems but yellow leaves, you may assign these as dominant traits. If you observe that some of your offspring plants have purple stems and green leaves, you might assume these are recessive traits. Based on your observations, create a testable hypothesis. You'll want to try to guess the father plant's stem and leaf colors based on your hypothesis.
  6. Intermate the plants using a bee stick or Q-tip. To do this, gently swap the bee stick on one plant, ensuring that the plant has pollen, and then sharing it with another plant. Do this several times to make sure that each plant receives pollen from several other plants, both with like and unlike observable traits. Do this once a day for three days.
  7. Once the three days are over, cut off any flower buds that were not pollinated.
  8. Stop watering the plants and let them dry out.
  9. Harvest the seeds and replant them, essentially starting the process again. These seeds are the second generation of offspring, or O2.
  10. Make observations about the stem and leaf color of the next generation of plants. Do you think your hypothesis was correct?
  11. Plant the yellow-green leaf seeds. These will be known as the 'father' or P2.
  12. After a few days, observe the stem and leaf colors of the P2 plants. Do your observations support your hypothesis?

Video Directions

This video shows how to do genetics labs and will help you tackle the procedure for studying the genetics of your plants.

Online Labs

It's worth noting that if growing peas and making homemade apparatuses are a bit more than you were bargaining for, there are a few great interactive labs online.

Mendel's Peas

This online lab is a replica of Mendel's pea experiments. The lab has a handy menu so you can actually explore the lab before doing anything. The lab takes you through various steps including planting the peas, observing their traits, and then cross pollinating the first plants you grew. This is exactly what Mendel did so students can get a feel for the tedious process he went through to come up with his observations.

Pea Soup

While graphically not as exciting, Pea Soup is another online option that helps students observe two traits in pea plants. To get started, you click on the 'begin experiment' button. Then you are brought to a page where you can choose to 'mate' two different peas. Their genotypes are written for you. Then the page will show you all the options available for the 'parents' you selected. The page moves quickly, and you can miss it if you're not writing everything down.

MIT's STAR Genetics

MIT's STAR Genetics lab is a downloadable 'game' of sorts where students can mix and match genotypes of a variety of species including pea plants, fruit flies, and even cows. The program is best suited towards high school students who have a strong understanding of Biology.

Genetics Is Fun

Whether you study pea plants or fruit flies, or simply go home and observe your parents' traits and try to figure out how you got your own, studying genetics can be a lot of fun. While modern genetics identifies a few things that Mendel got wrong, his theories still apply where traits are not linked or influenced by other factors.

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Gregor Mendel's Pea Plant Experiment