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How did Mendel discover the principles of heredity?

Mendel's success depended on several things:

  •  he picked a simple system (pea plants) that reproduced rapidly. 

  •  he  kept records on the traits of each plant and then took pollen from one plant to fertilize the flowers from another plant and saw predictable patterns in the offspring plants. 

What did Mendel discover?

Try to see these results as Mendel did, not knowing what is happening to cause the results. For example, Mendel first grew purebred lines of green peas and yellow peas. Then he took pollen from a green pea plant, smeared it on the flowers of a yellow pea plant, and saw that the new peas were always green.

What happened to the yellow trait? Did the traits that control yellow somehow disappear? How about the possibility that they were still there in the offspring but their influence was hidden by the more dominant green trait (see table below)? Mendel suspected that the trait for green was dominant over that for yellow because some of the next generation's peas were yellow! And this occurred in a consistent ratio of three green pea plants for every yellow one.

Why did this happen?

Things only made sense to Mendel if he assumed that each trait had TWO "controllers," and that the sex cells carried only one of the two controllers. Assuming that, Mendel  could make a chart like the one below:

Gene Possibility Chart

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