From Seed to Seed:
Plant Science for K-8 Educators
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From Seed to Seed: |
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Inheritance of Seed Color First, we'll need a few terms to get us started. A trait is a characteristic, such as eye color or seed color. The way a trait shows up is termed its expression. For example, you might say that for a person with brown eyes, the gene for brown eye color is expressed. Brown eyes are one expression of eye color; blue eyes are another expression. In the same manner, yellow and green are the two expressions of the gene for seed color. To begin his experiments, Mendel created groups of "pure parent" plants. These are plants whose offspring consistently possess a given trait through successive generations. For the experiment on pea seed color, Mendel selected one group of plants that consistently produced offspring with only yellow-seeded peas-that is, in which yellow was expressed. At the same time, he selected a second group of plants that always produced offspring with green-seeded peas. Once he had his "pure parents, " Mendel crossed the two groups of plants. He took pollen from one of the groups and pollinated flowers on the other group, and vice versa. He continued to care for the plants until they developed fruit-the pea pods. Once the enclosed seeds were mature, he harvested and planted them, and let these new plants grow to maturity. Then he carefully recorded his observations about the color of their seeds. These first offspring, produced by crossing two pure parents, are termed the F1-short for first filial-generation. It would be reasonable to guess that approximately one-half of the offspring would have yellow seeds, and one-half of the offspring would have green seeds-similar to the "heads or tails" flip of a coin. Of course, as you have probably guessed, things are never that simple! All of the offspring produced by these crosses had yellow seeds. This experiment was repeated with hundreds of plants, and the results were consistent. In other words, it wasn't just chance-as is the case if you toss a coin three times, and it lands heads up all three times. The odds of the coin landing on heads are 50-50, and if you toss the coin enough times, the results will eventually approximately reflect these odds. Imagine your surprise, however, if you tossed a coin hundreds of times, and it always landed heads up! Furthermore, Mendel found that these results were consistent with those of his other experiments. The offspring of a cross between two pure lines (the F1 generation) for a given trait always exhibited just one expression of the trait. |
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