THE LIVING WORLD
Unit Three. The Continuity of Life
10. Foundations of Genetics
Mendel experimented with a variety of traits in the garden pea and repeatedly made similar observations. In all, Mendel examined seven pairs of contrasting traits as shown in table 10.1. For each pair of contrasting traits that Mendel crossed he observed the same result, shown in figure 10.3, where a trait disappeared in the F1 generation only to reappear in the F2 generation. We will examine in detail Mendel’s crosses with flower color.
The F1 Generation
In the case of flower color, when Mendel crossed purple and white flowers, all the F1 generation plants he observed were purple; he did not see the contrasting trait, white flowers.
Mendel called the trait expressed in the F1 plants dominant and the trait not expressed recessive. In this case, purple flower color was dominant and white flower color recessive. Mendel studied several other characters in addition to flower color, and for every pair of contrasting traits Mendel examined, one proved to be dominant and the other recessive. The dominant and recessive traits for each character he studied are indicated in table 10.1.
TABLE 10.1 SEVEN CHARACTERS MENDEL STUDIED IN HIS EXPERIMENTS
The F2 Generation
After allowing individual F1 plants to mature and self-fertilize, Mendel collected and planted the seeds from each plant to see what the offspring in the F2 generation would look like. Mendel found (as Knight had earlier) that some F2 plants exhibited white flowers, the recessive trait. The recessive trait had disappeared in the F1 generation, only to reappear in the F2 generation. It must somehow have been present in the F1 individuals but unexpressed!
At this stage Mendel instituted his radical change in experimental design. He counted the number of each type among the F2 offspring. He believed the proportions of the F2 types would provide some clue about the mechanism of heredity. In the cross between the purple-flowered F1 plants, he counted a total of 929 F2 individuals (see table 10.1). Of these, 705 (75.9%) had purple flowers and 224 (24.1%) had white flowers. Approximately one-fourth of the F2 individuals exhibited the recessive form of the trait. Mendel carried out similar experiments with other traits, such as round versus wrinkled seeds (figure 10.4) and obtained the same result: Three-fourths of the F2 individuals exhibited the dominant form of the character, and one-fourth displayed the recessive form. In other words, the dominant:recessive ratio among the F2 plants was always close to 3:1.
Figure 10.4. Round versus wrinkled seeds.
One of the differences among varieties of pea plants that Mendel studied was the shape of the seed. In some varieties the seeds were round, whereas in others they were wrinkled.
A Disguised 1:2:1 Ratio
Mendel let the F2 plants self-fertilize for another generation and found that the one-fourth that were recessive were true- breeding—future generations showed nothing but the recessive trait. Thus, the white F2 individuals described previously showed only white flowers in the F3 generation (as shown on the right in figure 10.5). Among the three-fourths of the plants that had shown the dominant trait in the F2 generation, only one-third of the individuals were true-breeding in the F3 generation (as shown on the left). The others showed both traits in the F3 generation (as shown in the center)—and when Mendel counted their numbers, he found the ratio of dominant to recessive to again be 3:1! From these results Mendel concluded that the 3:1 ratio he had observed in the F2 generation was in fact a disguised 1:2:1 ratio:
Figure 10.5. The F2 generation is a disguised 1:2:1 ratio.
By allowing the F2 generation to self-fertilize, Mendel found from the offspring (F3) that the ratio of F2 plants was one true-breeding dominant, two not-true-breeding dominant, and one true-breeding recessive.
Key Learning Outcome 10.2. When Mendel crossed two contrasting traits and counted the offspring in the subsequent generations, he observed that all of the offspring in the first generation exhibited one (dominant) trait, and none exhibited the other (recessive) trait. In the following generation, 25% were true-breeding for the dominant trait, 50% were not-true-breeding and appeared dominant, and 25% were true-breeding for the recessive trait.