From Seed to Seed: |
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Dominant or Recessive?
Another surprise! About one-fourth of the offspring had green seeds! How can yellow-seeded parents produce green-seeded offspring? These results were again consistent with the results of his experiments on other traits. In every case, he found that approximately one-fourth of the plants in the F2 generation exhibited the other expression of the trait-the expression that didn't show up in the F1 generation. From these results, as well as the results from experiments on other plant traits over the course of 10 years, Mendel came to a remarkable conclusion-remarkable in its creativity as well as its accuracy. Mendel concluded that there were two "hereditary factors" for every plant trait studied. Furthermore, he postulated that one of the hereditary factors was dominant over the other one. He termed the trait that was consistently expressed in the first crossing (the F1 generation) the dominant trait. The trait that was not expressed in the F1 generation, but that reappeared in the F2 generation, he called the recessive trait. Mendel further theorized that during the formation of egg and sperm, these hereditary factors are segregated from one another. The parent plant has two hereditary factors, but only one of each pair goes to each gamete produced. Then, during fertilization, two gametes unite. The egg donates one hereditary factor; the sperm the other, so the resulting zygote, like the parent plants, contains two hereditary factors. Mendel concluded that a recessive factor can be carried through a generation unchanged, even when it is not expressed. This is called Mendel's Principle of Segregation, and explains how a trait can be hidden, but not lost. We'll use some diagrams to make things clearer. |
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