Lesson Explainer: Mendel’s Experiments Science

In this explainer, we will learn how to describe the experiments that Gregor Mendel performed on his pea plants and explain how his discoveries led to him writing his laws of inheritance.

Gregor Mendel was an Austrian monk who lived in the 1800s. He conducted simple experiments on pea plants to describe the major genetic inheritance principles. At the time, the scientific community rejected his ideas, and not until these ideas were rediscovered in the 1900s was his work taken seriously. Nowadays, he is considered the father of genetics.

Mendel was an avid gardener and was curious about how traits, variations in an organism’s characteristics, are inherited. He was specifically interested in hereditary traits, traits that are passed down from parents to offspring, For example, traits like height, eye color, and natural hair color are hereditary traits.

Definition: Hereditary Trait

A hereditary trait is a variation of a characteristic inherited from an organism’s parents.

This is in contrast to acquired traits, which occur from the interaction with one’s environment, such as muscle growth after lifting weights.

Definition: Acquired Trait

An acquired trait is a variation of a characteristic acquired from the interaction of an organism with its environment.

Example 1: Identifying Hereditary Traits in Humans

Some traits are passed from parents to offspring. An example of this is that a child may have the same eye color as one of their parents. What term is given to these traits?

  1. Determined
  2. Acquired
  3. Characteristic
  4. Dominant
  5. Hereditary

Answer

Traits are variations in observable characteristics. Examples of traits include a person’s specific eye color, skin color, height, and blood type. Although eye color is a characteristic, a quality or feature belonging to a particular organism, a more specific term applies. Therefore, “characteristic” is false.

Some traits result from an organism’s interaction with its environment and are not inherited from the organism’s parents. These traits include muscle size after lifting weights and calluses on hands and feet from manual labor. Such traits are called acquired traits. As eye color does not result from interactions with the environment, it is not an acquired trait. Therefore, “acquired” is incorrect.

Some traits mask other traits. For example, when a person with brown eyes has a child with a person with blue eyes, the child will most likely have brown eyes. This occurs because the brown eye trait masks the blue eye trait within the child. This relationship between traits is called dominance. The brown eye trait is dominant to the blue eye trait. Although some eye colors are dominant to others, not enough information is provided to determine whether the child’s eye color is dominant. Therefore, “dominant” is incorrect.

As some traits are acquired from interactions with the environment, other traits are passed down within families. Traits like eye color, height, and natural hair color are inherited from an individual’s parents. As these traits are inherited, they are hereditary traits.

Therefore, “hereditary” is correct.

In the 1800s, other scientists theorized that an organism’s characteristics are a blend of the traits of its mother and father. For example, they theorized that a cross between red and white flowers would result in pink flowers, as you can see in Figure 1.

Mendel noticed that while the blending hypothesis is sometimes true, it was not always true. Some traits would skip generations, and some traits would not even blend with other traits. For example, short plants crossed with tall plants would not result in an intermediate phenotype, a medium-height plant. Sometimes the offspring resulting from such a cross would be tall, and sometimes the offspring would be short.

Definition: Trait

A trait is a variation of a characteristic.

Definition: Offspring

Offspring are the newborn of living organisms, produced either by a single organism or, in the case of sexual reproduction, by two organisms.

Mendel decided to conduct experiments on pea plants to investigate the mechanisms of inheritance. He chose pea plants for specific reasons. Pea plants grow quickly and are easy to look after. He could grow many pea plants in his garden and have them quickly reach sexual maturity in the matter of weeks. Imagine if he had chosen to experiment on trees. It would have taken years until the trees reached sexual maturity, and they would have been hard to grow and manage. His experiments would have taken decades.

Pea plants also have obviously contrasting traits, such as smooth or wrinkled peas and green or yellow peas. This allowed Mendel to easily study these traits between generations.

Lastly, pea plants can self-pollinate or cross-pollinate or be artificially pollinated. The pea flower contains both female and male parts (named carpel and stamen, respectively), and thus pea plants are said to be hermaphrodite.

Key Term: Hermaphroditism

Hermaphroditism is when an organism has both male and female reproductive organs.

This is why pea plants can self-pollinate. They can also cross-pollinate with other plants and be artificially pollinated, as we will describe here. During self-pollination, the pea plant fertilizes itself to produce genetically identical offspring. Pea plants can also cross-pollinate, a process in which the pea plant is fertilized by another pea plant’s pollen to produce genetically unique offspring. Finally, pea plants can be artificially pollinated, a process in which a human fertilizes a pea plant with another chosen plant’s pollen. This gave Mendel control over which plants reproduced and allowed him to precisely design his experiments.

Example 2: Describing Mendel’s Reasons for Using Pea Plants in His Inheritance Experiments

Which of the following is not a reason that Mendel chose pea plants for his experiments?

  1. Pea plants can self-pollinate or be artificially pollinated by humans, so Mendel could investigate the effect of self- and cross-pollination.
  2. Pea plants grow quickly, so Mendel could produce many of them in a short amount of time.
  3. Pea plants have pairs of contrasting traits; for example, their pods can be yellow, or they can be green.
  4. Pea plants are very rare, so Mendel knew no one else was doing these experiments.

Answer

Mendel was a 19th-century monk who conducted experiments on pea plants to study how traits are passed down between generations.

Mendel specifically chose pea plants for his experiments for several reasons. Pea plants are easy to grow and mature quickly. He wanted to study how traits are passed down over generations. As pea plants grow very quickly and reach sexual maturity in the matter of weeks, Mendel was able to shorten his experiment time. If he had chosen another species that grows very slowly, it would have taken years for an organism to reproduce and produce offspring, making Mendel’s experiments incredibly long.

Also, peas have obviously contrasting traits that are easy to observe and study. For example, the seed and seed pod color can be green or yellow, the seed texture can be wrinkled or smooth, and the plant itself can be short or tall. This made trait distribution and variation easy to track and study within the experiments.

Lastly, pea plants can self-fertilize (a plant’s pollen fertilizes its own egg, generating genetically identical plants) or cross-pollinate (a plant’s pollen fertilizes another plant’s egg, generating genetically different plants). Mendel used this to his advantage. He was able to choose which plants to breed with other specific plants, adding calculated control to his experiments.

Pea plants are common garden plants. They are not rare. Mendel chose pea plants because they are easy to grow, have a fast generation time, have contrasting traits, and offer the ability to control which two plants to breed together. It should be noted, however, that Mendel was not the first to use pea plants for scientific purposes, as some other scientists had experimented on pea plants before him.

Using this information, let’s take a look back at our answer options. We can see that only one option is an incorrect reason for Mendel choosing pea plants, which is that pea plants are very rare, so Mendel knew no one else was doing these experiments.

Mendel artificially pollinated his pea plants in his experiments. He would carefully remove the stamen, which produces pollen granules containing sperm, from the plants that he wanted to cross. As pea plants can self-pollinate, removing the stamen prevents this from occurring. He would then use a paintbrush to brush pollen from one plant’s stamen onto another plant’s carpel, which contains the plant’s egg cells. The process that Mendel used to artificially pollinate plants is shown in Figure 2.

Definition: Self-Pollination

Self-pollination occurs when pollen grains containing the males gametes (sex cells) fertilize the female gametes (sex cells) of the same plant.

Definition: Cross-Pollination

Cross-pollination occurs when pollen grains containing the males gametes (sex cells) of one plant fertilize the female gametes (sex cells) of a different plant.

Example 3: Describing the Reasoning behind Mendel’s Experimental Steps in Pea Plants

Mendel carried out experiments to discover how the color of seeds was determined in pea plants. When he had a plant that was ready to use in his experiment, he removed the stamens from its flowers. Why?

  1. To prevent the plant from producing seeds
  2. To allow him to sequence the DNA of the plant
  3. To ensure the plant could not cross-pollinate
  4. To provide more space for seed growth
  5. To ensure this plant did not self-pollinate

Answer

Mendel, a monk who pioneered in genetic experiments, performed his experiments on pea plants in the 1800s. He meticulously studied how traits are inherited between pea plant generations before the discovery of DNA and is considered the father of genetics. As his work predates the discovery of DNA sequencing, the option referring to this experiment step allowing him to sequence the plant’s DNA is false.

Mendel specifically examined how traits like height, seed color, and seed texture are passed between pea plant generations. As plants generate from seeds, Mendel did not alter his pea plants to prevent seed production. In fact, he needed his plants to produce seeds to see what traits and characteristics their offspring carried. Therefore, the option that states that he prevented the plants from producing seeds is also false.

Mendel conducted his experiments by crossing specific plants with other plants. To do this, he artificially pollinated plants and controlled which plant’s sperm fertilizes a specific plant’s egg cells. The process in which one plant’s egg is fertilized by another plant’s sperm is called cross-pollination. As Mendel was crossing plants via cross-pollination to study their offspring’s traits, the option stating that he wanted to prevent cross-pollination is false.

Stamens are the male fertilizing organ of the plant. They contain anthers, small structures that produce pollen granules. Within these pollen granules are the plant’s sperm, which can fertilize other plants. Removing the stamens does not allow for more seed growth. Therefore, the option that suggests otherwise is also false.

Pea plants are self-fertilizing, meaning that the sperm in their pollen granules can fertilize their own egg cells located in the plant’s carpel, the female organ of the plant. The pea plant can pollinate itself, generating offspring that are genetically identical to the parent plant.

Mendel wanted to prevent self-fertilization, as he meticulously controlled which specific plants were fertilized. If he had allowed self-fertilization, it would have been difficult to draw conclusions on how traits are passed down to the offspring. Therefore, to control his experiments, Mendel removed the stamens to make sure that the plant does not have any pollen to fertilize itself.

Thus, the correct answer is to ensure this plant did not self-pollinate.

In Mendel’s first experiment, he examined the seed color in peas. Some pea plant seeds are yellow and some are green. Mendel wanted to check whether the yellow- and green-seeded plants exhibit true breeding, meaning that their offspring would always have the same seed color as their parents’ seed color. To do this, Mendel first self-pollinated plants with yellow seeds and plants with green seeds.

He self-fertilized the plants by applying each plant’s pollen to its own stigma. Consequently, each plant’s egg cell was fertilized with its own sperm, and the resulting seed carried only genetic information identical to its parent. Consequently, all self-fertilized yellow-seeded plants had only yellow-seeded offspring, and all self-fertilized green-seeded plants had only green-seeded offspring. This allowed Mendel to ensure that his pea plants were true-breeding plants. These true-breeding plants were the parent plants, as shown in Figure 3.

This first cross between the true-breeding parent plants is called the P cross, and all the resulting offspring from the P crosses are called the first filial or F generation. Things became even more interesting when Mendel crossed the true-breeding parent yellow-seeded plants with the true-breeding F green-seeded plants in the parent cross.

As you can see in Figure 4, the resulting F offspring did not have a yellow-green seed color as predicted by the blending theory. Rather, all the offspring had yellow seeds. The green trait disappeared.

However, when an F yellow-seeded plant was crossed with another yellow-seeded F plant to produce the F generation, the green trait reappeared.

In the F generation, when Mendel crossed many F yellow-seeded plants, about 75 out of 100 of the offspring had yellow seeds, but 25 out of the 100 of the offspring had green seeds. Mendel described these numbers with ratios. For every four plants, three plants had yellow seeds and one plant had green seeds. Therefore, the offspring had a yellow-to-green seed ratio of 31.

Mendel concluded from this experiment that some traits are dominant and some are recessive. Dominant traits always appear if the organism inherits the genetic information that encodes the trait. Meanwhile, recessive traits may disappear during cross-pollination.

Key Term: Dominant Trait

A dominant trait is the version of a trait that is always shown if the organism inherits the genetic information encoding that trait.

Key Term: Recessive Trait

A recessive trait is the version of a trait that is only shown if the genetic information that produces the dominant trait is absent.

From his experiment results, Mendel proposed the laws of inheritance.

The first law of inheritance that Mendel postulated is the law of independent assortment. This law states that unit factors that encode hereditary factors (these are now called genes) segregate independently from each other during gamete formation. Therefore, when an offspring inherits a seed color trait, this is independent of whether the organism inherits other traits, such as flower color or height.

Definition: Gene

A gene is a sequence of DNA that contains the information needed to produce a certain characteristic (e.g., eye color).

Mendel’s second law, the law of dominance, states that one trait version will conceal another trait version if the organism has two different trait versions. In this case, the two gene versions are said to be heterozygous and the organism is said to be a hybrid. Instead of the trait versions blending together as postulated by the blending theory of inheritance to produce a new, intermediate trait, the dominant trait is exhibited exclusively over the other version of the trait, which is said to be recessive. For instance, in pea plants, the trait of yellow seeds is dominant to the trait of green seeds. Many traits follow the law of dominance, and certain trait versions are dominant to other trait versions. In case an organism receives two versions of the same trait, this pair of genes is said to be homozygous (which means they are similar). This is the only case where the recessive trait can be shown.

Finally, Mendel postulated the law of segregation, which states that an organism inherits two copies of each hereditary factor (now known as genes). One copy is inherited from the organism’s mother, and the other copy is inherited from the organism’s father. For instance, in pea plants, an offspring may inherit one gene for yellow seeds from the mother plant and one gene for green seeds from the father plant.

Example 4: Describing Trait Version Types for Seed Color

The diagram shows how Mendel combined genetic material from a plant that produced yellow seeds with a plant that produced green seeds. He observed that all the plants produced only yellow seeds. What term can be used to describe the trait of yellow seeds?

Answer

Mendel, a monk who lived in the 1800s, performed experiments on pea plants to understand how traits are passed down between generations. He artificially pollinated plants and specifically crossed plants with certain traits to see what traits their offspring would carry.

In one experiment, Mendel crossed a plant that produced yellow seeds with a plant that produced green seeds. All the resulting offspring plants from the cross had yellow seeds. This generation was called the F generation. When two yellow-seeded F plants were crossed with each other, the green trait reappeared. Some resulting plants had yellow seeds, and some had green seeds.

From the results of this experiment, Mendel proposed the law of dominance. This law states that hybrid organisms (like a pea plant generated from both a yellow-seeded and a green-seeded plant) will only display the trait version that is dominant and will mask the recessive, or hidden, trait version. Here, the green seed trait is recessive because it becomes hidden in the presence of the yellow seed trait. As the yellow seed trait is displayed in the hybrid organism, it is a dominant trait.

Therefore, the term that can be used to describe the trait of yellow seeds is dominant.

Mendel’s experiments on pea plants were instrumental to understanding how traits are passed from the parents to the offspring and would later act as the foundation for the modern field of genetics.

Key Points

  • Mendel experimented on pea plants because they are easy to grow, grow very quickly, and have obviously contrasting traits and because he could precisely control their reproduction through self-pollination or cross-pollination.
  • Based on his experiments, which produced first- and second-generation pea plants, Mendel stated three important laws for genetic inheritance:
    • The law of dominance, which states that dominant traits mask recessive traits in hybrid organisms.
    • The law of independent assortment, which states that organisms inherit traits independently of other traits.
    • The law of segregation, which states that an organism inherits two hereditary factors (genes) per each trait: one copy inherited maternally and the other copy inherited paternally.

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