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Evolutionary Theory

Author(s): Tadzia GrandPré, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD

Mendel's Model of Particulate Inheritance

In contrast to Darwin's observational methods, Gregor Mendel used an experimental approach to examine questions about heredity. Interested in the fundamental question of how traits are passed from one generation to the next, Mendel spent eight years tracking the inheritance of specific traits through multiple generations of garden pea plants (Pisum sativum), in a series of studies that involved more than 28,000 plants.

For his studies, Mendel took advantage of the fact that pea plants are easy to breed and have distinctive and observable physical traits (phenotypes), including height, pea color and pea texture. He established pure breeding lines for these, and other phenotypes. These pure breeding lines are known as the parental, or P lines. As first steps in his experiments, Mendel crossed two different P lines, for example, plants with green peas and plants with yellow peas. He found that all of the offspring (known as the first filial, or F1 line) looked like only one of the parental lines; they all produced yellow peas. The green pea trait of the second parental line had disappeared.

As the next step, he crossed the yellow-pea offspring (the F1 line) produced by his original cross. The offspring produced by this cross (known as the second filial, or F2 line) had a combination of phenotypes: ¾ of the F2 offspring looked exactly like the previous F1 generation; they had yellow peas. However, the remaining ¼ of the F2 progeny displayed the green-pea trait of the original, parental line, which had been lost in the F1 generation. Thus, variation was restored. Based on the results of his experiments, Mendel proposed that parents pass on discreet particles, or factors (what we now call genes), to their offspring. 

To learn more about Mendel's work, review the presentation entitled "Introduction to Mendelian Genetics."