Question

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.)

Short answer

Answer: The gene interaction observed in this experiment is Incomplete Dominance. The genotypes for the three different fruit shapes in the F2 generation are AABB, AABb, AaBB, and AaBb (disc-shaped); AAbb and aaBB (spherical); and aabb (long).

Step-by-step solution

(a) Identifying the gene interaction type

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.

(b) Determining the number of gene pairs and assigning genotypes

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).

Key concepts

Incomplete Dominance

Incomplete dominance is a fascinating type of genetic interaction where neither allele is completely dominant over the other. Unlike complete dominance, where one allele masks the expression of another, incomplete dominance results in a blending of traits. This means the offspring exhibit a phenotype that is intermediate between the two parent phenotypes.

In the exercise with summer squash, the disc-shaped and long fruits crossed to produce an F1 generation displaying only disc-shaped fruits. However, when these F1 plants were crossed, the F2 generation showed three distinct phenotypes: disc-shaped, spherical, and long fruits. The phenotypic ratio was 9:6:1, deviating from the typical Mendelian 9:3:3:1.

• This deviation indicates incomplete dominance, as the phenotypes do not align with simple dominance or recessive patterns.
• It shows how two alleles can produce a variety of traits due to their interactions.

Incomplete dominance highlights the complexity of genetic inheritance beyond the straightforward Mendelian principles.

Phenotypic Ratios

The phenotypic ratio in genetics refers to the relative numbers of different phenotypes produced in offspring. In Mendelian genetics, a typical dihybrid cross results in a 9:3:3:1 ratio. However, deviations from this ratio suggest other forms of genetic interactions.

For summer squash, the 9:6:1 phenotypic ratio observed in the F2 generation indicates a more complex interaction. This pattern signifies that:

• 9 parts of the offspring are disc-shaped, meaning this phenotype involves at least one dominant allele from each of the gene pairs involved (A and B).
• 6 parts have spherical fruits, suggesting a mixed gene interaction where one dominant and one recessive allele are present in each pair.
• 1 part shows long fruits, indicating the recessive phenotype with both alleles being recessive.

Such ratios help geneticists understand the underlying genetic interactions and complexities beyond simple dominance and recessive rules.

Genotype and Phenotype

Genotype refers to the genetic makeup of an organism—the specific alleles present at a gene locus. Meanwhile, phenotype describes the physical expression or traits observed, which are influenced by the genotype.

In our exercise, understanding genotype and phenotype relationships is crucial to explaining the fruit shapes of summer squash:

• Disc-shaped fruits correspond to genotypes with at least one dominant allele in each gene pair: AABB, AaBB, AABb, AaBb.
• Spherical fruits have intermediate phenotypes, with one dominant and one recessive allele per gene pair, resulting in genotypes like AAbb or aaBB.
• Long fruits are recessive, with genotypes consisting entirely of recessive alleles: aabb.

By dissecting the genotype combinations and their resultant phenotypes, one can better comprehend the genetic architecture underlying these traits. Phenotypic expressions are thus bulk interpretations of underlying genotype combinations and are key areas of study in genetics.