Gregor Mendel

Key Terms

Mendel’s Pea Plant Experiments

Blending Inheritance

Before Gregor Mendel’s discoveries, the blending inheritance was a widely popular theory to explain heredity.

• The blending inheritance is a model where the offspring would have a mix of characteristics that the parents have. This model did not explain some of Mendel’s observations.
• Example: According to blending inheritance, a tall plant and a short plant would produce an offspring of medium height. If this were true, by now all humans would be of the same height!

Characteristics of Pea Plants

To understand his observations, Mendel experimented with pea plants. The pea plants were good experimental subjects because they grow quickly, are easy to raise, and have several visible characteristics that exit in two different forms. For example, the flower color may be either white or purple. The seven characteristics that Mendel studied are shown in the diagram below.
In Mendel’s pea plant experiments, Mendel needed to control pollination, the fertilization step in the sexual reproduction of plants.

• Pea plants have both the male flower part(anther)and the female flower part(stigma).They naturally self pollinate(a flower fertilizes itself).
• In order to study the offspring of two different parent plants, Mendel prevented self-pollination by removing the anthers of some pea plants.
• Mendel cross-pollinated by dusting the stigma with the pollen from the anther of a different pea plant.
• The resulting offspring is a hybrid.

Figure: Diagram showing
that seven characteristics
observed by Mendel

Mendel’s First Set of Experiments

In his first set of experiments, Mendel experimented with just one characteristic (such as flower color) at a time

Experimental Steps

1. Mendel allowed each variety of pea plants to self-pollinate for several generations. He wanted to ensure that the offspring of each variety would display the same characteristics.

•  Example: a pea plant with purple flowers will only produce plants with purple flowers
  
• These pea plants formed the P generation.

2. Mendel cross-pollinated two P generation plants with different characteristics.

• Example: a pea plant with purple flowers and a pea plant with white flowers
  
• The offspring formed the F₁ generation.

3. The plants of the F₁ generation were allowed to self-pollinate.

• The offspring formed the F₂ generation.
  

Results

• All the F₁ generation plants were alike.
• When plants in the F₁ generation are self-pollinated, the plants in the F₂ generation are no longer the same (exhibit two different characteristics).
• Furthermore, the F₂ generation plants is played the two different characteristics in a consistent ratio (3:1).
• Example: on average, there were three purple-flowered plants to one white-flowered plant

Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0

P generation stands for parental generation. F generation stands for filial generation (filial means ”of a son or daughter”).

Mendel’s Second Set of Experiments

In his second set of experiments, Mendel experimented with two characteristics at a time, such as a yellow round seeds with green wrinkled seeds. He wanted to know if different characteristics are inherited together. For example, were yellow seeds always round and green seeds always wrinkled? The experimental steps were the same as the ones in the first set of experiments.

See biology study guide Mendelian Inheritance to learn how to predict an offspring’s characteristic.

Results

• All the plants in the F₁ generation were the same.
• When plants in the F₁ generation were self-pollinated, the F₂ generation plants showed all possible combinations of the two characteristics.
• The F₂ generation plants displayed all possible combinations in a consistent ratio (9:3:3:1).

Mendel’s Conclusions

Notes