Passage III

Flower color in a species of snapdragon plant is determined by a single gene with two alleles. The allele for red flowers (R) is dominant to the allele for white flowers (r). A plant with the genotype RR or Rr will have red flowers, while a plant with the genotype rr will have white flowers.

When two heterozygous plants (Rr) are crossed, the expected phenotypic ratio of offspring is 3 red-flowered plants to 1 white-flowered plant. This ratio can be predicted using the equation:

Expected number = Total offspring × Expected proportion

The Punnett square below shows how alleles from each parent combine to produce the genotypes seen in the offspring.

Two experiments were performed to examine the inheritance of flower color under different environmental conditions.

Experiment 1

True-breeding red-flowered plants (RR) were crossed with true-breeding white-flowered plants (rr). The 200 offspring of this cross were labeled the F₁ generation. The F₁ plants were allowed to self-fertilize, and the resulting 200 offspring were labeled the F₂ generation. This process was repeated for two additional generations (F₃ and F₄) with 200 offspring for each generation. Table 1 shows the genotypes of the parent plants used in each cross.

The flower colors of the offspring were recorded for each generation. The results are shown in Figure 1.

Experiment 2

The procedure from Experiment 1 was repeated, but the F₂ plants were separated by phenotype before self-fertilizing. Group A consisted of red-flowered F₂ plants, and Group B consisted of white-flowered F₂ plants. Plants in each group were allowed to self-fertilize, and the flower colors of 150 offspring from each group were recorded. The results are shown in Figure 2.

Figure 2