Aresearcher has crossed two dihybrid fish with the genotype d+/d ; t+/t. he notices that the progeny of this cross demonstrate a phenotypic ratio of 9: 7. what type of gene interaction do these results suggest?
a. dominant epistasis
b. suppression
c. recessive epistasis
d. the two genes do not interact in any way.
e. the two genes act in the same pathway.

The correct answer is C: recessive epistasis. Double 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".

Observing the altered phenotype proportions of the offspring can reveal which interaction is operating in the trait determination.

In simple recessive epistasis, the proportion is 9:3:4. But in "double recessive epistasis", the phenotypic proportion is 9:7. In this case, both recessive alleles in any of both loci, are capable of suppressing the phenotype. Those individuals that present two recessive alleles for any loci have the same phenotype. These are dd / T --, D --/tt, dd/tt.

                   DT                       Dt                        dT                     dt

DT            DDTT                  DDTt                  DdTT                DdTt

Dt             DDTt                   DDtt                   DdTt                  Ddtt

dT             DdTT                  DdTT                 ddTT                 ddTt

dt              DdTt                  Ddtt                  ddTt                 ddtt

Phenotypic proportion: 9/16 (D-T-)

                                       7/16  (D-tt, ddT-, ddtt)

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