Students explore the concept of inherited traits and understand the significance of Gregor Mendel's discoveries related to heredity. Grades 3-5

chromosome: a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes

dominant gene: a gene that can hide the effect of a recessive gene

gene: a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring

heredity: the passing of traits from a parent to its offspring

hybrid: the offspring of two plants or animals of different species or varieties

inherited: derive a quality or characteristic genetically from one's parent or ancestors

offspring: the child or young of two parents

organism: any living thing made of cells

purebred: the offspring produced by parents of the same breed

recessive gene: a gene whose expression can be hidden

species: a group of organisms sharing common traits that can produce offspring together

trait: observable, physical characteristic obtained through genetic inheritance

In the 1850s, an Austrian monk named Gregor Mendel started a series of experiments with garden peas. Mendel was fascinated with plants and was curious about why some pea plants had different physical characteristics than others. For more than ten years, Mendel carried out thousands of experiments on pea plants that laid the foundation for the study of heredity.

At the time of Mendel’s studies, it was a generally accepted belief that traits were blended. It was believed that the offspring of a tall parent and a short parent would have medium-sized height. Mendel’s work showed that traits are not blended; they are passed on intact. He discovered that traits can skip a generation and are either dominant or recessive. Mendel also discovered that traits can be passed on independently of other traits. For example, the size of a pea plant does not affect the color of the plant’s flower. The importance of Mendel’s discoveries went largely unrecognized until the early 1900s when other scientists, who made many of the same observations, rediscovered his work.

Mendel’s decision to study peas is significant. Pea plants are a good choice for study because they produce a large number of offspring, and it is easy to control their pollination. Pea plants also have many traits that exist in only two forms. Mendel selected seven characteristics to study; purple or white flower color, flowers positioned on the top or the side of the stem, smooth or pinched seed pod, yellow or green pod color, yellow or green seed color, round or wrinkled seeds, long or short stems. Mendel studied each trait and learned how they were passed down to the offspring plant.

Gregor Mendel was the first scientist to determine that the mathematic principles of probability can be used to predict the outcomes of genetic crosses. In 1905, English geneticist Reginald Punnett created what is now known as the Punnett square to illustrate some of Mendel’s discoveries. A Punnett square is a chart that shows all possible gene combinations in a cross. This visual representation of Mendelian inheritance was designed as a teaching tool and helps explain how the laws of probability apply to genetics.

For each inherited trait, the offspring has two genes­—one from each parent. If both genes are either dominant or recessive, the trait is called a purebred trait. If only one gene is dominant, the combination is called a hybrid trait. The Punnett square to the right shows a cross between two hybrid tall pea plants. T represents the dominant gene for tall and t represents the recessive gene for short. When two hybrid tall pea plants are crossed, three-fourths of the plants are tall, and one-fourth are short. You can predict that there is a 75% probability the offspring will be tall and a 25% probability the offspring will be short.

Mendel’s discovery has had, and continues to have, a great impact on agricultural development. Understanding Mendelian Genetics enables animal and plant breeders to produce new, improved, and higher yielding varieties with more accuracy. These genetic advances, in addition to improvements in farm management, have led to significant increases in agricultural productivity.

1. Ask the students what a trait is. Allow students to offer their answers and guide them to the correct answer. Traits are distinguishing characteristics. Explain that many traits are determined by genetics.
2. Ask the students to list different plants or animals that a farmer might raise. List them on the board. Students could list livestock such as cattle, sheep, or pigs as well as plants such as wheat, corn, fruits, and vegetables.
3. Using the list on the board, ask students if there are any specific traits a farmer would want to have in their plants or animals. Allow students time to think about the question and then give them an example. For example if they named a strawberry in step 2, explain that strawberry farmers want their strawberries to be juicy, sweet, and medium sized. Each of these characteristics are traits that are influenced or determined by the plant's genetics. After giving the example, see if students can think of any more traits for the remaining farm crops or livestock.
4. Explain that farmers use their knowledge of inherited traits to try to produce the best product.

Activity 1: The Friar Who Grew Peas

1. Discuss the information found in the Background Agricultural Connections section of the lesson. You may want to use the Peas in a Pod Powerpoint Slides or excerpts from the book Gregor Mendel: The Friar Who Grew Peas by Cheryl Barbadoe.
2. Make a Punnett square model on the board to illustrate the results of Mendel’s experiments with pea plants. Yellow pompoms represent the dominant gene for yellow seeds and green pompoms represent the recessive gene for green seeds. Use two yellow pompom magnets for Parent 1 and two green pompom magnets for Parent 2. In this case, we are crossing two parents with purebred traits. When a purebred pea plant with yellow seeds is crossed with a purebred pea plant with green seeds, there is a 100% probability that the offspring will have yellow seeds. If this F1 generation (the first filial generation resulting from a cross between the first set of parents) is crossed, the results will be different.
3. Discuss the meaning of the Punnett square with the students. Write the following on the board to explain how dominant and recessive genes are expressed: When two dominant genes are present, the dominant gene will be expressed. When a dominant and a recessive gene are present, the dominant gene will be expressed. When two recessive genes are present, the recessive gene will be expressed.
4. Make another Punnett square model on the board to illustrate the F2 generation (the second filial generation resulting from a cross between two individuals from the F1 generation). Use one yellow and one green pompom magnet for Parent 1 and one yellow and one green pompom magnet for Parent 2. In the F2 generation, we are crossing two parents with hybrid traits. When a hybrid pea plant with yellow seeds is crossed with another hybrid pea plant with yellow seeds, there is a 75% probability that the offspring will have yellow seeds and a 25% probability that the offspring will have green seeds. It is important to note that in the case of a hybrid, the dominant trait is what will be expressed.

Activity 2: Pompom Punnett Squares

1. Fill lunch-size paper bags with 12 yellow and 12 green pompoms. Divide students into groups of four. Provide each student with a Punnett Square activity sheet and each group with a Punnett Square Chart and a bag of yellow and green pompoms.
2. In each group, students will take turns being Parent 1 and Parent 2. Each parent will close their eyes and choose two pompoms from the bag. They will then begin creating a Punnett square on their chart by placing their pompoms in the space provided for Parent 1 and for Parent 2. The group will use the remaining pompoms to fill out the rest of the Punnett square.
3. Each student will record the results on their Punnett Square activity sheet using a Y to represent yellow seeds and a g to represent green seeds. Underneath each Punnett square, students can record the probability of the offspring having yellow or green seeds.
4. This process can be repeated as many times as desired by choosing new pompoms from the bag. Pompoms can be replaced with other props to represent different pea plant traits, such as tall and short pipe cleaners for stem height. Use the Pea Plant Traits Chart to determine whether the traits are dominant or recessive when creating Punnett squares for different traits.

After conducting these activities, review and summarize the following key concepts:

• Gregor Mendel carried out experiments on pea plants that laid the foundation for the study of heredity.
• Punnett squares are a tool used to predict the traits of an offspring. A Punnett square is a chart that shows all possible gene combinations in a cross. This visual representation of Mendelian inheritance was designed as a teaching tool and helps explain how the laws of probability apply to genetics.
• Farmers use their knowledge of genetics and biology to select ideal traits in the crops or livestock they produce. Understanding Mendelian genetics enables animal and plant breeders to produce new, improved, and higher yielding varieties with more accuracy. These genetic advancements, in addition to improvements in farm management, have led to significant increases in agricultural productivity.

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