Overview: What Were Mendel’s Experiments?
Mendel conducted experiments with simple garden pea plants in order to study plant breeding. His experiments were so successful because the plants grew quickly and self-pollinated, ensuring that traits would “breed true” and appear rapidly in successive generations. He kept careful and systematic records, and studied traits that had obvious contrasts. The results formed the basics of heredity.
Seven Contrasting Traits
Mendel chose to study a limited number of traits that were either expressed one way or its opposite. The seeds themselves were either round or wrinkled, yellow or green; their coating, colored or white; and the pods, inflated or constricted, green or yellow. The flowers on the pea plants were either in the middle of the stem or at the end of the stem; and the stem length of the entire plant was either tall or short. He studied individual traits systematically to see how they varied in each of three generations.
Types and Genes
Pea plants self-pollinate, so that generations are identical. Tall plants will produce more tall plants; yellow-seeded plants will produce more yellow-seeded plants; and so on. Mendel crossbred plants with contrasting traits by carefully pollinating a tall parent plant with pollen from a short plant. In the first generation (F1), however, all the plants were tall. In the second generation (F2), three-fourth of the plants were tall, but one-fourth were short. Mendel called the traits “factors”, and they are now called “genes”.
Dominance
In each of the seven pairs of traits that Mendel examined, one trait appeared in the first generation, but its opposite did not appear until the second generation. For example, suppose a plant with green pods was pollinated by a plant with yellow pods. In the first generation, all plants would have green pods. Not until the second generation would one-fourth of the plants have yellow pods. Mendel believed that the dominant trait prevented its opposite from appearing in some way.
Segregation and Independent Assortment
Mendel showed in his experiments that the members of each pair of genes separated when gametes formed, so that the offspring would carry traits that were expressed in one generation, but hidden in the next. He also proposed that traits such as roundness, color, and all the rest were separate from one another in the principle of independent assortment. One plant could have green pods and round seeds, while another could have green pods and wrinkled seeds. Similarly, another could have yellow pods and round seeds, or yellow pods and wrinkled seeds.
Cell Theory and Heredity
Many of Mendel’s ideas were not accepted until after other biological discoveries were made. For example, chromosomes had not yet been discovered. The process of meiosis, which explains the principle of segregation, was observed thirty years after his experiments were finished. The principle of independent assortment was formed, because Mendel chose traits to study that were on separate chromosomes that would vary independently.
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