8.2: Laws of Inheritance

Of Peas and People

These purplish-flowered plants are not just pretty to look at. Plants like these led to a huge leap forward in biology. The plants are common garden peas, and they were studied in the mid-1800s by an Austrian monk named Gregor Mendel. With his careful experiments, Mendel uncovered the secrets of heredity, or how parents pass characteristics to their offspring. You may not care much about heredity in pea plants, but you probably care about your own heredity. Mendel's discoveries apply to people as well as to peas — and to all other living things that reproduce sexually. In this concept, you will read about Mendel's experiments and the secrets of heredity that he discovered.

Figure \(\PageIndex{1}\): Sweet Pea flower

Mendel and His Pea Plants

Gregor Mendel, shown below, 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 pea plants like the one pictured above.

Figure \(\PageIndex{2}\): 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!

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 vary. These characteristics, some of which are illustrated in Figure \(\PageIndex{3}\). Each of these characteristics has two common traits ( values).

1. Seeds can be round or wrinkled
2. Seeds can have yellow or green cotyledons. Cotyledons refer to the tiny leaves inside the seeds.
3. Flowers can be white or violet
4. The seed pod can be full or constricted
5. The seed pod can be yellow or green
6. The flowers can occur along the stem (in axial pods) or at the end of a stem (in terminal pods)
7. Stems can be long (6-7 feet) or short (less than 1 foot).

Figure \(\PageIndex{3}\): Mendel investigated seven different characteristics in pea plants.

Controlling Pollination

Figure \(\PageIndex{4}\): 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.

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 sex cells, or gametes, of plants. They are produced by a male flower part called the anther (Figure \(\PageIndex{4}\)). 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.

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. When the term hybrid is used in this context, it refers to any offspring resulting from the breeding of two genetically distinct individuals.

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 \(\PageIndex{5}\), Mendel cross-pollinated violet-flowered and white-flowered parent plants. The parent plants in the experiments are referred to as the P (for parent) generation.

F1 and F2 Generations

Figure \(\PageIndex{5}\): In one of his experiments on inheritance patterns, Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color (the P generation). The resulting hybrids in the F1 generation all had violet flowers. In the F2 generation, approximately three-quarters of the plants had violet flowers, while one-quarter had white flowers.

The offspring of the P generation are called the F1 (for filial, or “offspring”) generation. As shown in Figure \(\PageIndex{5}\), all of the plants in the F1 generation had violet 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 violet 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 violet 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 violet-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 \(\PageIndex{5}\).

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 round seeds like one of the two parents. When the F1 generation plants were 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.

Figure \(\PageIndex{6}\): The parent generation consisted of smooth yellow peas and green wrinkled peas. The first offspring generation all had smooth yellow seeds. When these F1 plants were crossed, the offspring had Smooth yellow, smooth green, wrinkled yellow and wrinkled green seeds in a ratio of 9:3:3:1.

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 shown in the 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 Legacy

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? Because Mendel's work was largely ignored. Mendel was far ahead of his time and working from a remote monastery. He had no reputation among the scientific community and limited previously published work. He also published his research in an obscure scientific journal. As a result, when Charles Darwin published his landmark book on evolution in 1869, although Mendel's work had been published just a few years earlier, Darwin was unaware of it. Consequently, Darwin knew nothing about Mendel's laws and didn’t understand heredity. This made Darwin's arguments about evolution less convincing to many people.

Then, in 1900, three different European scientists — named DeVries, Correns, and Tschermak — independently arrived at Mendel's laws. All three had done experiments similar to Mendel's and come to the same conclusions that he had drawn several decades earlier. Only then was Mendel's work rediscovered and Mendel himself given the credit he was due. Although Mendel knew nothing about genes, which were discovered after his death, he is now considered the father of genetics.

Review

1. What is the blending theory of inheritance? What observations led Mendel to question this theory?
2. Why were pea plants a good choice for Mendel's experiments?
3. Describe Mendel's first set of experiments, including the results.
4. State Mendel's two laws of inheritance.
5. How did the outcome of Mendel's second set of experiments lead to his second law?
6. Discuss Mendel's legacy.
7. In Mendel’s first set of experiments:
   1. Why did he use pea plants with different characteristics for the parental generation?
   2. Why do you think he only tested one characteristic at a time?
   3. Why did he allow the plants in the F1 generation to self-pollinate?
   4. If he observed 200 F2 plants, approximately how many would have purple flowers? Approximately how many would have white flowers? Explain your answers.
   5. Which flower color seemed to dominate over the other? Explain your answer.
8. If Mendel’s law of independent assortment was not correct, and characteristics were always inherited together, what types of offspring do you think would have been produced by crossing plants with yellow round seeds and green wrinkled seeds? Explain your answer.
9. True or False. In Mendel’s experiments, the F1 generations are hybrids.
10. True or False. A single gamete of a pea plant contains factors that result in both a purple flower and a white flower.

Explore More

https://bio.libretexts.org/link?16761#Explore_More

Every mother and father pass down traits to their children. Explore how Mendel's pea plant experiments helped us better understand the genetics of this process here:

Attributions

1. Sweet pea flower by Giligone licensed CC BY-SA 3.0 via Wikimedia Commons
2. Gregor Mendel by William Bateson, public domain via Wikimedia Commons
3. Mendel seven characteristics by Mariana Ruiz LadyofHats, released into the public domain via Wikimedia Commons
4. Flower structure by OpenStax, CC BY 4.0
5. Mendel's experiments by CNX, CC BY 4.0
6. Pea cross by Suzanne Wakim, licensed CC BY 4.0 adapted from on Dihybrid Cross by CNX OpenStax, licensed CC BY 4.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0