Two plants with white flowers, each from true-breeding strains, were crossed. All the F1 plants had red flowers. When these F1 plants were intercrossed, they produced an F2 consisting of 177 plants with red flowers and 142 with white flowers. (a) Propose an explanation for the inheritance of flower color in this plant species. (b) Propose a biochemical pathway for flower pigmentation and indicate which genes control which steps in this pathway.

(a) This is an example of double recessive epistasis

(b) The proposed pathway is in the explanation and in the attached files

Explanation:

Gene interactions happen between genes at different levels and have implications in phenotypic expression, causing a wide genetic variability.  

Observing the altered phenotype proportions of the offspring can reveal which interaction is operating in the trait determination. So now, let us focus on identifying the interaction.

In the exposed situation, the whole F1 is red-flowered, but from the F2 177 plants have red flowers and 142 have white flowers.

So we know that the total number of individuals is 177 + 142 = 319

We know that from the cross between two white-flowered plants, only red-flowered plants are produced. The only way of getting this result (red-flowered plants from two white-flowered plants) is by the interaction of two or more genes. So let us assume that there are two genes involved in this process. A cross between two dihybrids individuals will give 16 possible genotypes, so:

319 plants --------------------------- 16

177 plants with red flowers---------X = 8.87 ≅ 9

142 plants with white flowers------X = 7.13 ≅ 7

The phenotypic ratio 9:7 corresponds to double recessive epistasis.

Epistasis means "interruption" and refers to interactions between genes located in different loci in the same chromosome. An “epistatic gene” can alter, influence, or suppress the expression of a "hypostatic gene". When the epistatic gene is recessive, the interaction is known as "recessive epistasis".

In duplicate recessive epistasis, two genes are involved in the same metabolic path. Two recessive alleles in any of the two loci can suppress or inhibit the phenotype. Those individuals that present two recessive alleles for any loci have the same phenotype.

In this example let us say that there are two genes codifying for the color of the flowers, genes A and B. The white phenotype (absence of pigments) is due to the presence of recessive alleles in any loci. Individuals with the genotypes aaB-, A-bb, and aabb will be white-flowered. On the contrary, individuals with the genotype A-B- will be red-flowered.

To produce the red color, a plant needs the A compound to turn to the B compound (controlled by gene A), and the B compound to turn to the C compound (controlled by gene B). Finally, the C compound produces RED

Compound A turns to compound B through the action of an enzyme. Let us name it enzyme 1. Compound B passes to compound C through the action of another enzyme, which we will name enzyme 2. At least one dominant allele in the first locus is necessary to produce enzyme 1. And at least one dominant allele is necessary to produce enzyme 2. That is why the red color is only visible if both dominant alleles are present. If at least one of them is missing, white will be expressed (absence of pigments).  

                        Genotype A-                               Genotype B-

Compound A --> Enzyme 1 --> Compound B --> Enzyme 2 --> Compound C

                                                                                                       (RED COLOR)

Option 1: Genotype aa. Enzyme 1 can not be produced and compound A    

                can not turn to B. The flower is white.

                        Genotype aa                               Genotype B-

Compound A --> Enzyme 1  ║  Compound B --> Enzyme 2 --> Compound C

                           (WHITE)

Option 2: Genotype bb. Enzyme 2 can not be produced and compound B    

                can not turn to C. The flower is white.

                        Genotype A-                               Genotype bb

Compound A --> Enzyme 1 --> Compound B --> Enzyme 2  ║ Compound C

                                                                                  (WHITE)              

Option 3: Genotype aa + bb. Enzyme 1 can not be produced and compound A can not turn to B. Enzyme 2 can not be produced and compound B can not turn to C. The flower is white

                        Genotype aa                               Genotype bb

Compound A --> Enzyme 1 ║ Compound B --> Enzyme 2 ║ Compound C

                             (white)                                       (white)