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Mendel's First Set of Experiments

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Mendel's Laws: The Law of Segregation

Peas. Some round and some wrinkled. Why?

That's what Mendel asked. He noticed peas were always round or wrinkled, but never anything else. Seed shape was one of the traits Mendel studied in his first set of experiments.

Mendel’s First Set of Experiments

Mendel first experimented with just one characteristic of a pea plant at a time. He began with flower color. As shown in Figure below , Mendel cross-pollinated purple- and white-flowered parent plants. The parent plants in the experiments are referred to as the P (for parent) generation . You can explore an interactive animation of Mendel’s first set of experiments at this link: http://www2.edc.org/weblabs/Mendel/mendel.html .

Parental, F1, and F2 generations of peas

This diagram shows Mendel's first experiment with pea plants. The F1 generation results from cross-pollination of two parent (P) plants, and contained all purple flowers. The F2 generation results from self-pollination of F1 plants, and contained 75% purple flowers and 25% white flowers.

F1 and F2 Generations

The offspring of the P generation are called the F1 (for filial, or “offspring”) generation . As you can see from Figure above , all of the plants in the F1 generation had purple flowers. None of them had white flowers. Mendel wondered what had happened to the white-flower characteristic. He assumed some type of inherited factor produces white flowers and some other inherited factor produces purple flowers. Did the white-flower factor just disappear in the F1 generation? If so, then the offspring of the F1 generation—called the F2 generation —should all have purple flowers like their parents.

To test this prediction, Mendel allowed the F1 generation plants to self-pollinate. He was surprised by the results. Some of the F2 generation plants had white flowers. He studied hundreds of F2 generation plants, and for every three purple-flowered plants, there was an average of one white-flowered plant.

Law of Segregation

Mendel did the same experiment for all seven characteristics. In each case, one value of the characteristic disappeared in the F1 plants and then showed up again in the F2 plants. And in each case, 75 percent of F2 plants had one value of the characteristic and 25 percent had the other value. Based on these observations, Mendel formulated his first law of inheritance. This law is called the law of segregation . It states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces.

Summary

  • Mendel first researched one characteristic at a time. This led to his law of segregation. This law states that each characteristic is controlled by two factors, which separate and go to different gametes when an organism reproduces.

Making Connections

Practice I

Use these resources to answer the questions that follow.

  1. Define a true-breeding strain. How did Mendel make sure the plants were true-breeding?
  2. What is a monohybrid cross?
  3. How was the F 2 generation formed in Mendel's experiments?
  1. Describe Mendel's key observations.
  2. Define allele. Give an example.
  3. What is the difference between homozygous and heterozygous?
  4. Why did Mendel not observe any white flowered plants in the F 1 generation of his experiment?
  5. Why was Mendel able to observe white flowered plants in the F 2 generation of his experiment?
  6. How many alleles of a gene are in a gamete?
  7. Explain Mendel's Law of Segregation.

Practice II

Review

1. Describe in general terms Mendel’s first set of experiments.

2. State Mendel's first law.

3. Assume you are investigating the inheritance of stem length in pea plants. You cross-pollinate a short-stemmed plant with a long-stemmed plant. All of the offspring have long stems. Then, you let the offspring self-pollinate. Describe the stem lengths you would expect to find in the second generation of offspring.

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