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6.1: Mendel's Investigations

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Lesson Objectives

  • Explain why and how Mendel studied pea plants.
  • Describe the results of Mendel's experiments.
  • State Mendel's laws of segregation and independent assortment.
  • Outline the genetics of inheritance.

Vocabulary

allele
one of two or more different versions of the same gene
dominant allele
allele that masks the presence of another allele for the same gene when they occur together in a heterozygote
genetics
the science of heredity
genotype
alleles an individual inherits at a particular genetic locus
heterozygote
organism that inherits two different alleles for a given gene
homozygote
organism that inherits two alleles of the same type for a given gene
hybrid
offspring that results from a cross between two different types of parents
law of independent assortment
Mendel’s second law stating that factors controlling different characteristics are inherited independently of each other
law of segregation
Mendel’s first law stating that the two factors controlling a characteristics separate and go to different gametes
locus
position of a gene on a chromosome
phenotype
characteristics of an organism that depend on how the organism’s genotype is expressed
pollen
tiny grains that bear the male gametes of seed plants and transfer sperm to female reproductive structures
pollination
fertilization in plants in which pollen is transferred to female gametes in an ovary
recessive allele
allele that is masked by the presence of another allele for the same gene when they occur together in a heterozygote

Introduction

People have long known that the characteristics of living things are similar in parents and their offspring. Whether it’s the flower color in pea plants or nose shape in people, it is obvious that offspring resemble their parents. However, it wasn’t until the experiments of Gregor Mendel that scientists understood how characteristics are inherited. Mendel's discoveries formed the basis of genetics, the science of heredity. That’s why Mendel is often called the “father of genetics.” It’s not common for a single researcher to have such an important impact on science. The importance of Mendel's work was due to three things: a curious mind, sound scientific methods, and good luck. You’ll see why when you read about Mendel's experiments.

An introduction to heredity can be seen at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/12/eEUvRrhmcxM (17:27).

Mendel and His Pea Plants

Gregor Mendel was born in 1822 and grew up on his parents’ farm in Austria. He did well in school and became a monk. He also went to the University of Vienna, where he studied science and math. His professors encouraged him to learn science through experimentation and to use math to make sense of his results. Mendel is best known for his experiments with the pea plant Pisum sativum (see Figure below). You can watch a video about Mendel and his research at the following link: http://www.metacafe.com/watch/hl-19246625/milestones_in_science_engineering_gregor_mendel_and_classical_genetics/.

Gregor Mendel. The Austrian monk Gregor Mendel experimented with pea plants. He did all of his research in the garden of the monastery where he lived.

Blending Theory of Inheritance

During Mendel's time, the blending theory of inheritance was popular. This is the theory that offspring have a blend, or mix, of the characteristics of their parents. Mendel noticed plants in his own garden that weren’t a blend of the parents. For example, a tall plant and a short plant had offspring that were either tall or short but not medium in height. Observations such as these led Mendel to question the blending theory. He wondered if there was a different underlying principle that could explain how characteristics are inherited. He decided to experiment with pea plants to find out. In fact, Mendel experimented with almost 30,000 pea plants over the next several years! At the following link, you can watch an animation in which Mendel explains how he arrived at his decision to study inheritance in pea plants: http://www.dnalc.org/view/16170-Animation-3-Gene-s-don-t-blend-.html.

Why Study Pea Plants?

Why did Mendel choose common, garden-variety pea plants for his experiments? Pea plants are a good choice because they are fast growing and easy to raise. They also have several visible characteristics that may vary. These characteristics, which are shown in Figure below, include seed form and color, flower color, pod form and color, placement of pods and flowers on stems, and stem length. Each characteristic has two common values. For example, seed form may be round or wrinkled, and flower color may be white or purple (violet).

Mendel investigated seven different characteristics in pea plants. In this chart, cotyledons refer to the tiny leaves inside seeds. Axial pods are located along the stems. Terminal pods are located at the ends of the stems.

Controlling Pollination

To research how characteristics are passed from parents to offspring, Mendel needed to control pollination. Pollination is the fertilization step in the sexual reproduction of plants. Pollen consists of tiny grains that are the male gametes of plants. They are produced by a male flower part called the anther (see Figure below). Pollination occurs when pollen is transferred from the anther to the stigma of the same or another flower. The stigma is a female part of a flower. It passes the pollen grains to female gametes in the ovary.

Flowers are the reproductive organs of plants. Each pea plant flower has both male and female parts. The anther is part of the stamen, the male structure that produces male gametes (pollen). The stigma is part of the pistil, the female structure that produces female gametes and guides the pollen grains to them. The stigma receives the pollen grains and passes them to the ovary, which contains female gametes.

Pea plants are naturally self-pollinating. In self-pollination, pollen grains from anthers on one plant are transferred to stigmas of flowers on the same plant. Mendel was interested in the offspring of two different parent plants, so he had to prevent self-pollination. He removed the anthers from the flowers of some of the plants in his experiments. Then he pollinated them by hand with pollen from other parent plants of his choice. When pollen from one plant fertilizes another plant of the same species, it is called cross-pollination. The offspring that result from such a cross are called hybrids.

Mendel's First Set of Experiments

At first, Mendel experimented with just one characteristic 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.

This diagram shows Mendel's first experiment with pea plants. The F1 generation results from cross-pollination of two parent (P) plants. The F2 generation results from self-pollination of F1 plants.

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.

Mendel's Second Set of Experiments

Mendel wondered whether different characteristics are inherited together. For example, are purple flowers and tall stems always inherited together? Or do these two characteristics show up in different combinations in offspring? To answer these questions, Mendel next investigated two characteristics at a time. For example, he crossed plants with yellow round seeds and plants with green wrinkled seeds. The results of this cross are shown in Figure below.

This chart represents Mendel's second set of experiments. It shows the outcome of a cross between plants that differ in seed color (yellow or green) and seed form (shown here with a smooth round appearance or wrinkled appearance). The letters R, r, Y, and y represent genes for the characteristics Mendel was studying. Mendel didn’t know about genes, however. Genes would not be discovered until several decades later. This experiment demonstrates that 9/16 were round yellow, 3/16 were wrinkled yellow, 3/16 were round green, and 1/16 were wrinkled green.

F1 and F2 Generations

In this set of experiments, Mendel observed that plants in the F1 generation were all alike. All of them had yellow and round seeds like one of the two parents. When the F1 generation plants self-pollinated, however, their offspring—the F2 generation—showed all possible combinations of the two characteristics. Some had green round seeds, for example, and some had yellow wrinkled seeds. These combinations of characteristics were not present in the F1 or P generations.

Law of Independent Assortment

Mendel repeated this experiment with other combinations of characteristics, such as flower color and stem length. Each time, the results were the same as those in Figure above. The results of Mendel's second set of experiments led to his second law. This is the law of independent assortment. It states that factors controlling different characteristics are inherited independently of each other.

Mendel's Laws and Genetics

You might think that Mendel's discoveries would have made a big impact on science as soon as he made them. But you would be wrong. Why? Mendel never published his work. Charles Darwin published his landmark book on evolution in 1869, not long after Mendel had discovered his laws, but Darwin knew nothing of Mendel's discoveries. As a result, Darwin didn’t understand heredity. This made his arguments about evolution less convincing to many people. This example shows why it is important for scientists to communicate the results of their investigations.

Rediscovering Mendel's Work

Mendel's work was virtually unknown until 1900. Then, in that year, three different European scientists—named DeVries, Correns, and Tschermak—independently arrived at Mendel's laws. All three had done experiments similar to Mendel's. They came to the same conclusions that he had drawn almost half a century earlier. Only then was Mendel's actual work rediscovered. As scientists learned more about heredity over the next few decades, they were able to describe Mendel's ideas about inheritance in terms of genes. In this way, the field of genetics was born. At the link that follows, you can watch an animation of Mendel explaining his laws of inheritance in genetic terms. http://www.dnalc.org/view/16182-Animation-4-Some-genes-are-dominant-.html

Genetics of Inheritance

Today, we known that characteristics of organisms are controlled by genes on chromosomes (see Figure below). The position of a gene on a chromosome is called its locus. In sexually reproducing organisms, each individual has two copies of the same gene. One copy comes from each parent. The gene for a characteristic may have different versions. The different versions are called alleles. For example, in pea plants, there is a purple-flower allele (B) and a white-flower allele (b). Different alleles account for much of the variation in the characteristics of organisms.

Chromosome, Gene, Locus, and Allele. This diagram shows how the concepts of chromosome, gene, locus, and allele are related. What is the different between a gene and a locus? Between a gene and an allele?

During meiosis, homologous chromosomes separate and go to different gametes. Thus, the two alleles for each gene also go to different gametes. At the same time, different chromosomes assort independently. As a result, alleles for different genes assort independently as well. In these ways, alleles are shuffled and recombined in each parent’s gametes.

Genotype and Phenotype

When gametes unite during fertilization, the resulting zygote inherits two alleles for each gene. One allele comes from each parent. The alleles an individual inherits make up the individual’s genotype. The two alleles may be the same or different. As shown in Table below, an organism with two alleles of the same type (BB or bb) is called a homozygote. An organism with two different alleles (Bb) is called a heterozygote.

Alleles Genotypes Phenotypes
BB (homozygote) purple flowers
B (purple) Bb (heterozygote) purple flowers
b (white) bb (homozygote) white flowers

Table 6.2 There are two alleles, B and b, that control flower color in pea plants. This results in three possible genotypes. Why are there only two phenotypes?

The expression of an organism’s genotype produces its phenotype. The phenotype refers to the organism’s characteristics, such as purple or white flowers. As you can see from Table above, different genotypes may produce the same phenotype. For example, BB and Bb genotypes both produce plants with purple flowers. Why does this happen? In a Bb heterozygote, only the B allele is expressed, so the b allele doesn’t influence the phenotype. In general, when only one of two alleles is expressed in the phenotype, the expressed allele is called the dominant allele. The allele that isn’t expressed is called the recessive allele.

Lesson Summary

  • Gregor Mendel experimented with pea plants to learn how characteristics are passed from parents to offspring.
  • First, Mendel 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.
  • Then Mendel researched two characteristics at a time. This led to his law of independent assortment. This law states that the factors controlling different characteristics are inherited independently of each other.
  • Mendel's work was rediscovered in 1900. Soon after that, genes and alleles were discovered. This allowed Mendel's laws to be stated in terms of the inheritance of alleles.
  • Gregor Mendel - From the Garden to the Genome can be viewed at http://www.youtube.com/watch?v=6OPJnO9W_rQ (30.23).

Lesson Review Questions

Recall

1. What is the blending theory of inheritance? Why did Mendel question this theory?

2. List the seven characteristics that Mendel investigated in pea plants.

3. How did Mendel control pollination in pea plants?

4. Describe in general terms Mendel's first set of experiments.

5. What was Mendel investigating with his second set of experiments? What was the outcome?

6. State Mendel's two laws.

Apply Concepts

7. 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.

8. If a purple-flowered, short-stemmed plant is crossed with a white-flowered, long-stemmed plant, would all of the purple-flowered offspring also have short stems? Why or why not?

Think Critically

9. If Darwin knew of Mendel's work, how might it have influenced his theory of evolution? Do you think this would have affected how well Darwin’s work was accepted?

10. Explain Mendel's laws in genetic terms, that is, in terms of chromosomes, genes, and alleles.

11. Explain the relationship between genotype and phenotype. How can one phenotype result from more than one genotype?

Points to Consider

With his first set of experiments, Mendel found that characteristics appear to skip generations. With his second set of experiments, he found that different characteristics are inherited independently of one another.

  • Why would this information be useful? Can you think of a practical application of Mendel's laws?
  • Could Mendel's laws be used to predict the characteristics of the offspring of a given set of parents? How do you think this might be done?

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