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Mendel's Pea Plants

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Mendel's Pea Plants

Vocabulary

  • cross-pollinate
  • dominant
  • F1 generation
  • F2 generation
  • gametes
  • genetics
  • inherit
  • offspring
  • recessive
  • self-pollinate


Why do you look like your family?

For a long time people understood that traits are passed down through families. The rules of how this worked were unclear, however. The work of Gregor Mendel was crucial in explaining how traits are passed down to each generation.

Mendel's Experiments

What does the word "inherit" mean? You may have inherited something of value from a grandparent or another family member. To inherit is to receive something from someone who came before you. You can inherit objects, but you can also inherit traits. For example, you can inherit a parent's eye color, hair color, or even the shape of your nose and ears!

Genetics is the study of inheritance. The field of genetics seeks to explain how traits are passed on from one generation to the next.

In the late 1850s, an Austrian monk named Gregor Mendel ( Figure below ) performed the first genetics experiments.

Gregor Mendel

Gregor Mendel, the "father" of genetics.

To study genetics, Mendel chose to work with pea plants because they have easily identifiable traits ( Figure below ). For example, pea plants are either tall or short, which is an easy trait to observe. Furthermore, pea plants grow quickly, so he could complete many experiments in a short period of time.

The Laws of Heredity

Characteristics of pea plants.

Mendel also used pea plants because they can either self-pollinate or be cross-pollinated . Self-pollination means that only one flower is involved; the flower's own pollen lands on the female sex organs. Cross pollination is done by hand by moving pollen from one flower to the stigma of another. As a result, one plant's sex cells combine with another plant's sex cells. This is called a "cross." These crosses produce offspring (or "children"), just like when male and female animals mate. Since Mendel could move pollen between plants, he could carefully control and then observe the results of crosses between two different types of plants.

He studied the inheritance patterns for many different traits in peas, including round seeds versus wrinkled seeds, white flowers versus purple flowers, and tall plants versus short plants. Because of his work, Mendel is considered the "Father of Genetics."

Mendel's First Experiment

In one of Mendel's early experiments, he crossed a short plant and a tall plant. What do you predict the offspring of these plants were? Medium-sized plants? Most people during Mendel's time would have said medium-sized. But an unexpected result occurred.

Mendel observed that the offspring of this cross (called the F1 generation ) were all tall plants!

Next, Mendel let the F1 generation self-pollinate. That means the tall plant offspring were crossed with each other. He found that 75% of their offspring (the F2 generation ) were tall, while 25% were short. Shortness skipped a generation. But why? In all, Mendel studied seven characteristics, with almost 20,000 F2 plants analyzed. All of his results were similar to the first experiment—about three out of every four plants had one trait, while just one out of every four plants had the other.

For example, he crossed purple flowered-plants and white flowered-plants. Do you think the colors blended? No, they did not. Just like the previous experiment, all offspring in this cross (the F1 generation) were one color: purple. In the F2 generation, 75% of plants had purple flowers and 25% had white flowers ( Figure below ). There was no blending of traits in any of Mendel's experiments.

The results of Mendel's experiment with purple flowered and white flowered-plants numerically matched the results of his experiments with other pea plant traits.

Dominance

Do you remember what happened when Mendel crossed purple flowered-plants and white flowered-plants? All the offspring had purple flowers. There was no blending of traits in any of Mendel's experiments. Mendel had to come up with a theory of inheritance to explain his results.

Mendel's Explanation

Mendel proposed that each pea plant had two hereditary factors for each trait. There were two possibilities for each hereditary factor, such as a purple factor or white factor. One factor is  dominant  to the other. The other trait that is masked is called the  recessive  factor, meaning that when both factors are present, only the effects of the dominant factor are noticeable. Although you have two hereditary factors for each trait, each parent can only pass on one of these factors to the offspring. When the sex cells, or  gametes  (sperm or egg), form, the heredity factors must separate, so there is only one factor per gamete. In other words, the factors are "segregated" in each gamete. Mendel's law of segregation states that the two hereditary factors separate when gametes are formed. When fertilization occurs, the offspring receive one hereditary factor from each gamete, so the resulting offspring have two factors.

Example Cross

This law explains what Mendel had seen in the F1 generation when a tall plant was crossed with a short plant. The two heredity factors in this case were the short and tall factors. Each individual in the F1 would have one of each factor, and as the tall factor is dominant to the short factor (the recessive factor), all the plants appeared tall.

In describing genetic crosses, letters are used. The dominant factor is represented with a capital letter ( T  for tall) while the recessive factor is represented by a lowercase letter ( t ). For the  T  and  t  factors, three combinations are possible:  TT Tt , and  tt TT  plants will be tall, while plants with  tt  will be short. Since  T  is dominant to  t , plants that are  Tt  will be tall because the dominant factor masks the recessive factor.

In this example, we are crossing a  TT  tall plant with a  tt  short plant. As each parent gives one factor to the F1 generation, all of the F1 generation will be  Tt  tall plants.

When the F1 generation ( Tt ) is allowed to self-pollinate, each parent will give one factor ( T  or  t ) to the F2 generation. So the F2 offspring will have four possible combinations of factors:  TT Tt tT , or  tt . According to the laws of probability, 25% of the offspring would be  tt , so they would appear short. And 75% would have at least one  T  factor and would be tall.

Summary

  • Gregor Mendel was the father of the field of genetics, which seeks to explain how traits are passed on from one generation to the next.
  • To study genetics, Mendel chose to work with pea plants because they have easily identifiable traits.

Practice

Use the resource below to answer the questions that follow.

  1. What is a "simple" trait?
  2. What is a heterozygote? How is this different than a homozygote?
  3. You breed a plant with yellow wrinkled peas with a plant with yellow smooth peas. Both individuals are homozygous for both traits. What will the peas of the next generation look like?
  4. You breed plants with the same traits as in question 3, but this time the smooth trait is heterozygous in the second individual. What will the peas of the next generation look like?
  5. Since not all individuals in a generation successfully reproduce, how do you think the size of a population affects its ability to maintain heterozygotes in the population? What would happen to a population that lost all its heterozygotes? Think carefully and explain your answer fully.

Review

  1. Why did Mendel choose to study pea plants?
  2. How did Mendel's experiments disprove the idea that we are simply a "blend" of our parents' traits?

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