Gregor Mendel and an Introduction to Genetics
7. Mendel's Second Experiments
Mendel’s Second Set of Experiments
Mendel wondered whether different characteristics are inherited together. For example, are purple flowers and tall stems always inherited together? Or do these two characteristics show up in different combinations in offspring? To answer these questions, Mendel next investigated two characteristics at a time. For example, he crossed plants with yellow full pods and plants with green constricted pods. The results of this cross are shown in Figure below.
This chart represents Mendels second set of experiments. It shows the outcome of a cross between plants that differ in pod color (yellow or green) and pod form (full [shown here with a smooth appearance] or constricted [wrinkled]). The letters R, r, Y, and y represent genes for the characteristics Mendel was studying. Mendel didn't know about genes, however. Genes would not be discovered until several decades later.
F1 and F2 Generations
In this set of experiments, Mendel observed that plants in the F1 generation were all alike. All of them had full yellow pods like one of the two parents. When the F1 generation plants self-pollinated, however, their offspring—the F2 generation—showed all possible combinations of the two characteristics. Some had green full pods, for example, and some had yellow constricted pods. These combinations of characteristics were not present in the F1 or P generations.
Law of Independent Assortment
Mendel repeated this experiment with other combinations of characteristics, such as flower color and stem length. Each time, the results were the same as those in Figure above. The results of Mendel’s second set of experiments led to his second law. This is the law of independent assortment. It states that factors controlling different characteristics are inherited independently of each other.
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