When summer squash plants (Cucurbita pepo) with discshaped fruits are crossed to ones with long fruits, the \(\mathrm{F}_{1}\) generation all have disc-shaped fruits. When the \(F_{1}\) plants are crossed to each other, the \(\mathrm{F}_{2}\) produce spherical fruits as well as exhibit the two parental strains. The phenotypic ratio is 9: 6: 1 (disc-shaped:spherical:long). (a) Which type of gene interaction is this an example of? (b) Explain the phenotypes observed in terms of the number of gene pairs involved and by designating genotypes for all the fruit shapes in the cross. (Use dashes where required.)

In this case, the phenotypic ratio in the F2 generation is 9: 6: 1 (disc-shaped:spherical:long). This deviates from the typical Mendelian ratio (9:3:3:1) in a dihybrid cross. This suggests that the gene interaction is not of the simple Mendelian type; rather, it is an example of gene interaction called Incomplete Dominance, where neither allele is fully dominant over the other.

Since there are three different phenotypes in the F2 generation, we can infer that there are two gene pairs involved in determining the fruit shape in these summer squash plants. We can represent these pairs by using two different letters (A and B). Let's use the capital letters (A and B) for the dominant alleles and lowercase letters (a and b) for the recessive alleles. Now, let's assign genotypes to each fruit shape in the cross: 1. Disc-shaped fruits: This shape is dominant over the other shapes, so the genotype must have at least one dominant allele in each gene pair. Therefore, the genotypes for disc-shaped fruits are AABB, AABb, AaBB, and AaBb. 2. Spherical fruits: This is an intermediate phenotype, where the two genes must work together to produce the shape. Thus, these plants must inherit one dominant and one recessive allele from each gene pair. The genotypes for spherical fruits are AAbb and aaBB. 3. Long fruits: This phenotype is recessive, so the genotype for these fruits must include only recessive alleles: aabb. In summary, the gene interaction observed in this experiment is Incomplete Dominance, and the genotypes for the three different fruit shapes are AABB, AABb, AaBB, and AaBb (disc-shaped); AAbb and aaBB (spherical); and aabb (long).