Question

Many species of plants provide an excellent example of epistasis, where multiple genes can control the phenotype. In the case of corn, there is a gene that controls color. The kernels can either be blue (B) or red (b). A different gene determines whether or not that color will actually appear in the kernel. The dominant allele (A) inhibits color, meaning the kernels will be white, while the recessive allele (a) allows color (either red or blue). If a plant that is heterozygous for both traits is self crossed, what will the resulting phenotypic ratios of the F1 generation be

Answer 1

In cases of double epistasis, the dominant allele of one of the involved genes inhibits the expression of the other gene. The phenotypic ratio of the offspring in the F1 generation is 12:3:1.

Available data

- Two genes are involved in color expression

- One of the genes controls the color

• The dominant allele B codes for blue kernels

• The recessive allele b codes for red kernels

- The other gene determines whether or not that color will actually appear

• The dominant allele A inhibits the color ⇒ kernels are white

• The recessive allele a allows the color expression

Since the dominant allele A is the one that inhibits the expression of the color, we can assume it is dominant epistasis.

What is dominant epistasis?

• When talking about epistasis, we are referring to an interruption.

• These are 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 dominant, the interaction is known as dominant epistasis.

• In dominant epistasis cases, the dominant epistatic allele inhibits the expression of the hypostatic gene.

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

• The ratio 12:3:1 belongs to dominant epistasis.

Now, let us perform the cross.

Cross: a plant that is heterozygous for both traits is self crossed

Parentals)         AaBb       x       AaBb

Gametes) AB, Ab, aB, ab    AB, Ab, aB, ab

Punnett 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

F1)   Genotypes

• 1/16 AABB
• 2/16 AABb
• 1/16 AAbb
• 2/16 AaBB
• 4/16 AaBb
• 2/16 Aabb
• 1/16 aaBB
• 2/16 aaBb
• 1/16 aabb

       Phenotypes

• 12/16 White kernels ⇒ AABB, AABb, AAbb, AaBB, AaBb, Aabb
• 3/16 Blue Kernels ⇒ aaBB, aaBb
• 1/16 Red kernels ⇒ aabb

The phenotypic ratio of the progeny is 12:3:1 → 12 white, 3 blue, 1 red.

You will learn more about epistasis at

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