In a particular species of tomato plant, the plants can bear fruits of different colors. Suppose a researcher crosses a homozygous plant that bears white fruit with a different stock of homozygous plant that also bears white fruit. In the F1 generation, all of the fruits are red. Two F1 plants are then crossed and the F2 progeny are examined. The F2 generation contains 219 plants with red fruit and 173 with white fruit. Required:
a. What are the genotypes of the F2 progeny?
b. What type of epistasis, if any, is present?

1. dominant epistasis
2. duplicate recessive epistasis
3. duplicate dominant epistasis
4. no epistasis
5. recessive epistasis

Explanation:

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

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, in this example, tomato color.

Tomatoes that express the absence of color in the fruit have the following genotypes: aa B_, A_bb y aabb. Tomatoes with these genotypes are white, while tomatoes with A_B_ genotypes are red.

Cross 1:

Parental) aaBB   x   AAbb

Gametes) aB  aB    Ab  Ab

Punnet Square)    aB          aB

                   Ab    AaBb     AaBb

                   Ab     AaBb     AaBb

F1) 100% AaBb, red tomatoes

Cross 2:

Parental)  AaBb    x    AaBb

Gametes) AB Ab aB ab

                AB Ab aB ab

Punnet Square)   AB      Ab      aB      ab

                 AB    AABB  AABb  AaBB  AaBb

                 Ab   AABb  AAbb  AaBb  Aabb

                 aB    AaBB  Aabb   aaBB  aaBb

                 ab    AaBb  Aabb   aaBb   aabb

F2)  9/16 A-B-

      3/16 A-bb

      3/16 aaB-

      1/16 aabb

Genotypes A-bb, aaB- and aabb express white or absence of color.

Genotypes A-B- express red color.    

We know that in the F2 generation there are 219 plants with red fruit and 173 with white fruit.

The total number of plants is 392 (219 + 173), which is 100% of the F2 generation. So:

392 plants 100% F2 16

219 red plants56% F2 9

173 white plants44% F2 7  

There is a  proportion of 9:7 because of the interaction between the two genes.            

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