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Mendel’s Experiment

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Sexual Reproduction and Genetics

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Lesson Focus

Mendel’s Experiment

Biology

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You will investigate the traits of pea plants as a result of cross-pollination with the same, or different, varieties.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that cross-pollination is the process in which pollen from one plant is transferred to another plant for fertilization.
  • Determine the appearance of the offspring after cross-pollination of a hybrid or pure plant.
  • Explain the terms dominant and recessive and describe how a pair of genes determines the appearance of a trait.

Everything You'll Have Covered

Heredity is an important concept in an area of biology called genetics. Gregor Mendel was one of the first scientists to document the inheritance of traits from parent to offspring. The plant he chose to use for his study was the pea plant. Mendel selected this plant in particular because it is easy to control its pollination and has many contrasting traits such as color, height, and seed shape that are controlled by a segment of DNA called a gene.

Some cells in a pea plant have half the amount of DNA compared to regular cells. These cells make up sex cells, such as sperm cells (found in pollen grains) and egg cells (found in the pistil). The DNA in sperm and egg cells contain one set of all of the genes from the parent plant. The sperm and egg cells combine to form a fertilized egg with two sets of genes, one set from each parent. This forms offspring with the complete amount of DNA required for a pea plant.

In the process of cross-pollination, the pollen grain is removed from the anther of one pea plant and is transferred to the pistil of another pea plant. The pollen grain sticks to the top of the pistil and begins to grow a tube that travels down into the pistil to the ovary. The pollen grain is made up of two sperm cells so the sperm cells travel down the tube to the ovary at the base of the pistil and combine with the egg cells in the ovary to form a seed.

Mendel studied simple dominant and recessive traits in pea plants and found that a dominant gene is determinant over the corresponding recessive gene. In simple dominant and recessive traits, the trait of the offspring will be either completely dominant or recessive. For instance, if a plant has two copies of the dominant gene, it is considered to be pure and will express the dominant trait. Similarly, if a plant has two copies of the recessive gene, it is considered to be pure and will express the recessive trait. However, if a plant has one copy of the dominant gene and one of the recessive, it will still express the dominant trait and is considered to be a hybrid. In normal situations, a hybrid will not express both traits equally or a blended mixture of the two. For example, a pea plant with a gene for a tall trait and a gene for a short trait will not express a medium trait.

Consequently, Mendel noticed that a pure dominant and pure recessive plant produced only hybrid dominant offspring. When the hybrids were crossed, the dominant and recessive traits were observed in a ratio of three to one. Therefore, there is a 25% chance that the two recessive genes will come together, producing 25% of the offspring with the recessive trait.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should be familiar with the terms pollination and flowers.
Course Biology
Type of Tutorial Experiment
Key Vocabulary alleles, chromosomes, cross-pollination