5.4 Mendel and His Peas

Learning Objectives

• Describe Mendel's first genetics experiments.

Introduction

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.

Guided Learning

What is Heredity?

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! Heredity is the passing of traits from parent to offspring.

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

Gregor Mendel

In the late 1850s, an Austrian monk named Gregor Mendel (Figure below) performed the first genetics experiments. Because of his work, Mendel is considered the "Father of Genetics."

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Gregor Mendel

Mendel studied the inheritance patterns for many different traits, or forms of characteristics, in pea plants, including round seeds versus wrinkled seeds, white flowers versus purple flowers, and tall plants versus short plants. He noticed that sometimes a trait would be present in one generation of pea plants but would disappear in the offspring of those plants. Mendel performed a variety of experiments on pea plants to try to understand what was happening. Pea plants were a great organism for Mendel to study 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.

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Characteristics of pea plants.

Pea plants were also a good choice for study because they can either self-pollinate or be cross-pollinated. Self-pollination means that only one flower is involved; the flower's pollen can fertilize its own ovule, or egg. Cross pollination occurs when pollen from one flower fertilizes the ovule of another plant. 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.

Mendel's Experiments

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 most of their offspring (the F2 generation) were tall, but a few were short. Shortness skipped the F1 generation, but reappeared in the F2 generation!

Mendel then repeated the same experiment for purple flowered-plants and white flowered-plants. When he crossed the purple 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, most of plants had purple flowers, and a few had white flowers. There was no blending of traits in any of Mendel's experiments. 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. Mendel called the trait that was seen in the F1 generation a dominant trait. The trait that disappeared in the F1 generation and then reappeared was called a recessive trait.

Review

• 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.
• Mendel discovered that some traits, which he called recessive, can skip a generation while other traits, which he called dominant, are present in all generations.

Vocabulary

Cross-Pollination

Movement of pollen from one flower to the stigma of another so that one plant's sex cells combine with another plant's sex cells.

Dominant Trait

Trait that was present in the F1 generation.

F1 Generation

Offspring of a cross between individuals that have contrasting traits.

F2 Generation

Offspring from the self-pollination of the F1 generation.

Genetics

Study of heredity.

Heredity

The passing of traits from parent to offspring.

Offspring

Result of a reproductive process; children.

Self-Pollination

Fertilization that occurs when a flower's pollen lands on it's own stigma and pollinates itself.

Recessive Trait

Trait that is hidden in the F1 generation.

Trait

The different forms of a characteristic in a population, such as brown hair or red hair.