Round and green, round and yellow, wrinkled and green, or wrinkled and yellow?
After his careful and extensive crosses in which Mendel experimented with 7 different traits one at a time, he began to wonder - Can two traits be inherited together? Or are all traits inherited separately? Answering this question could help answer the question of how traits were passed from parents to offspring.
Mendel’s Second Set of Experiments
After observing the results of his first 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 round seeds and plants with green wrinkled seeds. A cross which studies two traits at a time is called a dihybrid cross . The results of this cross, which is a dihybrid cross, are shown in Figure below .
This chart represents Mendel's second set of experiments. It shows the outcome of a cross between plants that differ in seed color (yellow or green) and seed form (shown here with a smooth round appearance or wrinkled appearance). 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. This experiment demonstrates that in the F2 generation, 9/16 were round yellow seeds, 3/16 were wrinkled yellow seeds, 3/16 were round green seeds, and 1/16 were wrinkled green seeds.
F1 and F2 Generations
In this set of experiments, Mendel observed that, just like his first set of experiments, the plants in the F1 generation were all alike. All of them had yellow and round seeds 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 round seeds, for example, and some had yellow wrinkled seeds. These combinations of characteristics were not present in the F1 or P generations. Once again, traits had somehow been carried in a hidden form in the F1 generation. Even though they weren't expressed (seen), they could still be passed along to offspring.
Repeating this experiment many times showed that once again, the F2 generation showed a predictable ratio of results in which 9/16 had round, yellow seeds : 3/16 had wrinkled, yellow seeds : 3/16 had round, green seeds : and 1/16 had wrinkled, green seeds.
How many seeds show the Round / Yellow trait? How many show the Round / Green train? The Wrinkled / Yellow trait? The Wrinkled / Green trait?
Law of Independent Assortment
Mendel repeated this experiment with other combinations of characteristics, such as flower color and stem length. Each time two different traits were tracked (dihybrid cross), the results showed the same 9:3:3:1 ratio 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. In other words, whether a pea plant is tall or short has nothing to do with what flower color it will have (purple or white).
If the original traits of the P generation were somehow linked together, the offspring of the F2 generation should show a 3:1 ratio. This did not happen. A 9:3:3:1 ratio proved that the traits were sorted independently.
- Mendel's second set of experiments crossed pea plants that differed in 2 different traits. This is called a dihybrid cross.
- Mendel found that all the offspring of the F1 generation were identical.
- The F2 generation showed all the possible combinations of traits in a predictable 9:3:3:1 ratio. Traits not seen in the F1 generation reappeared.
- Mendel proposed his second law of inheritance which he called the law of independent assortment. This law states that the factors controlling different characteristics are inherited independently of each other.