Of Peas and People

These purple-flowered plants are not just pretty to look at. Plants like these led to a huge leap forward in biology. They’re common garden peas, and they were studied in the mid-1800s by an Austrian monk named Gregor Mendel. Through careful experimentation, 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.

Gregor Mendel (Figure 5.10.2) was born in 1822. He grew up on his parents’ farm in Austria. He did well in school and became a friar (and later an abbot) at St. Thomas’ Abbey. Through sponsorship from the monastery, he went on 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 purple flower pictured in Figure 5.10.1).

Blending Theory of Inheritance

During Mendel’s time, the blending theory of inheritance was popular. According to this theory, offspring have a blend (or mix) of their parents’ characteristics. Mendel, however, 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 — 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 thousand 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 can vary. These characteristics — some of which are illustrated in Figure 5.10.4 — include seed form and colour, flower colour, pod form and colour, placement of pods and flowers on stems, and stem length. Each of these characteristics has two common values. For example, seed form may be round or wrinkled, and flower colour may be white or purple (violet).

To research how characteristics are passed from parents to offspring, Mendel needed to control pollination, which 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. 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, and it passes 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 using a small paintbrush 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.

At first, Mendel experimented with just one characteristic at a time. He began with flower colour. As shown in Figure 5.10.5, Mendel cross-pollinated purple- and white-flowered parent plants. The parent plants in the experiments are referred to as the P (for parent) generation.

Figure 5.10.5 shows Mendel’s first experiment with pea plants. The F1 generation results from the cross-pollination of two parent (P) plants, and it contains all purple flowers. The F2 generation results from the self-pollination of F1 plants, and contains 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 shown in Figure 5.10.5, 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 that 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, later showing up again in the F2 plants. In each case, 75 per cent of F2 plants had one value of the characteristic, while 25 per cent 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 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 5.10.6.

Figure 5.10.6 shows the outcome of a cross between plants that differ in seed colour (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, because genes would not be discovered until several decades later. This experiment demonstrates that, in the F2 generation, nine out of 16 were round yellow seeds, three out of 16 were wrinkled yellow seeds, three out of 16 were round green seeds, and one out of 16 was wrinkled green seeds.

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 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 colour and stem length. Each time, the results were the same as those shown in Figure 5.10.6. 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.

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 he was working from a remote monastery. He had no reputation in the scientific community and had only published sparingly in the past. Additionally, he published this 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.

Then, in 1900, three different European scientists — Hugo de DeVries, Carl Correns, and Erich von Tschermak — arrived independently 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, so that Mendel himself could be 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.

5.10 Cultural Connection

Corn is the world’s most produced crop.  Canada produces 13,000-14,000 metric Kilo tonnes of corn annually, mostly in fields in Ontario, Quebec and Manitoba.  Approximately 1.5 million hectares are devoted to this crop which is critically important for both humans and livestock as a food source.  Despite these high numbers of output, Canada is still only 11th on the list of world corn producers, with USA, China and Brazil claiming the top three places.  How did corn become such an important part of modern agriculture?

We didn’t always have corn as we know it.  Modern corn is descended from a type of grass called teosinte (Figure 5.10.7) native to Mesoamerica (southern part of North America).  It is estimated that Indigenous people have been harvesting corn and corn ancestors for over 9000 years. Excavations of the Xihuatoxtla Shelter in southwestern Mexico revealed our earliest evidence of domesticated corn: maize remains on tools dating back 8,700 years.

Ancient Indigenous peoples of southern Mexico developed corn from grass plants using a process we now call selective breeding, also known as artificial selection.   Teosinte doesn’t resemble the corn we have today- it had only a few kernels individually encased on very hard shells, and yet today we have multiple varieties of corn with row upon row of bare kernels.  This means that ancient agriculturalists among the Indigenous people of Mexico were intentionally cross-breeding strains of teosinte, and later, early maize to create plants which had more kernels, and reduced seed casings.  Watch the TED Ed video in the Explore More section to see what other changes agriculturalists have made to modern-day corn.

Maize-teosinte by John Doebley on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.

References

Brainard, J/ CK-12 Foundation. (2016). Figure 5 Mendel’s first experiment [digital image]. In CK-12 College Human Biology (Section 5.9) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-human-biology/section/5.9/

Brainard, J/ CK-12 Foundation. (2016). Figure 6 Mendel’s second experiment [digital image]. In CK-12 College Human Biology (Section 5.9) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-human-biology/section/5.9/

Junkyboss. (2016, March 31). 10 Strange hybrid fruits. YouTube. https://www.youtube.com/watch?v=ogc367xyzfk&feature=youtu.be

TED-Ed. (2013, March 12). How Mendel’s pea plants helped us understand genetics – Hortensia Jiménez Díaz. YouTube. https://www.youtube.com/watch?v=Mehz7tCxjSE&feature=youtu.be

TED-Ed. (2019, November 26). The history of the world according to corn – Chris A. Kniesly. YouTube. https://www.youtube.com/watch?v=i6teBcfKpik&feature=youtu.be

Wikipedia contributors. (2020, June 1). Carl Correns. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Carl_Correns&oldid=960172546

Wikipedia contributors. (2020, July 8). Charles Darwin. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Charles_Darwin&oldid=966652322

Wikipedia contributors. (2020, March 9). Erich von Tschermak. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Erich_von_Tschermak&oldid=944695823

Wikipedia contributors. (2020, July 7). Hugo de Vries. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Hugo_de_Vries&oldid=966513671