Genes interact with one other and control the phenotypes. Some of the genes not only control their own expression but also mask or regulate the expression of other genes. These genes, known as epistatic genes, mask the activity of the other genes. The epistatic genes create certain modifications in other genes. The genes getting masked or altered by the other genes do not express themselves due to the activity of masking. Hence, they are known as hypostatic genes.
Epistasis also results due to the presence of a single dominant allele in a pair. We intend to discuss the recessive epistasis, dominant epistasis, and epistasis involving duplicate genes. Mostly the F2 generations show the phenotypic ratio of 9:3:3:1.
Image: Examples of Epistasis (Coat color in the mice depicting the recessive epistasis with a phenotypic ratio of 9:3:4; Color of summer squash depicting the dominant epistasis with a phenotypic ratio of 12:3:1; and flower color depicting the epistasis involving duplicate genes with a phenotypic ratio of 9:7).
Recessive epistasis:
Unlike normal epistasis, the F2 generation shows a phenotypic ratio of 9:3:4. The recessive epistasis involves the individuals with one single dominant allele or two pairs of recessive alleles ( example A/- b/b and a/a b/ b). Let us consider an example of recessive epistasis in rodents. The coat color in rodents is a classic example of recessive epistasis. The wild-type mice exhibit a grayish coat color since their fur consists of a combination of black and yellow banding of hairs. This type of coloration is also known as agouti pattern of coat color. The Albino mice have a white coat color and pink eyes since they do not produce the pigment. Other variants are completely black. Consider a cross between the agouti mouse (A/A G/G) and an albino mouse (a/a g/g). The F1 progeny are all agouti mice (A/a G/g). Interbreeding of the agouti progeny or the F1 generation gives rise to F2 progeny consisting of 9 agouti mice ( A/- G/-), 3 black mice (a/a G/-), and 4 albino mice (g/g) or (A/- g/g). Hence the phenotypic ratio is 9:3:4. Black mice have dominant alleles and the albino mice have recessive alleles. In this case, the dominant allele (G) leads to pigment formation. The recessive allele (g allele) stops the expression of the pigment when present In a double dose or homozygous state. Also, the dominant allele for the agouti (A) helps in the expression of agouti coat color. The recessive allele (a) gives rise to no agouti color. Another example of recessive epistasis involves the coat color in the Labrador retrievers. Black, brown, and yellowish white coat colors are present in these species.
Dominant epistasis
Consider the individuals with A/- B/- phenotype and A/- b/b phenotype. Both of them exhibit the same phenotype. Hence, the phenotypic ratio is 12:3:1 in the F1 generation. Here, one dominant gene becomes epistatic to the other gene. This phenomenon is known as dominant epistasis. The fruit color of the summer squash is a classic example of dominant epistasis. The summer squash has three colors such as white, yellow, and green. White gets frequently expressed. Crosses involving white and yellow or white and green produce generations having white-colored fruits. In the crosses involving yellow and green, the yellow color dominates. Hence, yellow is recessive to white but is dominant to the green color. The theory behind the fruit coloration in summer squash involves a biochemical pathway having intermediate pigments. The product responsible for the white color gets yellow end product via an intermediate product responsible for the green coloration. Depending on the presence of the dominant allele the color of the fruit gets fixed.
Another example of dominant epistasis involves greeting horses. Various coat colors of horses are seen depending on the expression of the dominant gene.
Epistasis involving the duplicate genes:
Some genes express identical phenotypes or traits. Example, a phenotype expressed by one gene could be identical to the phenotype expressed by another gene. Such genes expressing the same phenotypes are known as duplicate genes.
Consider an example of sweet pea flower color. The purple coloration of the flower dominates over the white coloration. Meaning the pigment responsible for the production of the purple pigment becomes dominant. It gives a phenotypic ratio of 3:1 in the F2 generation. A cross involving white-flowered plants gives rise to the progeny plants all having purple flowers. Interbreeding these progeny plants result in the next generation of plants showing a phenotypic ratio of 9:7.
Consider two genes in the plant such as the C gene and the P gene. Both of them decide the flower coloration. These two genes interact with each other and impose a combined effect on the flower color. This phenomenon is known as complementary gene action or duplicate recessive epistasis.
Consider a biochemical pathway starting with the first compound giving rise to white pigmentation. Under certain enzymatic conditions, the white compound-1 results in the formation of white-compound-2. This step requires the expression of the C gene. The white-compound-2 further gives rise to a compound capable of promoting purple pigment. This step requires the expression of the P gene. The recessive homozygous alleles block this pathway. In such conditions, the P gene expression fails. As a result, only the white compound gets produced. Expression of both the genes becomes essential for the expression of the purple-colored phenotype.
Consider an example of duplicate dominant epistasis. The shepherd purse includes two types of fruits such as heart and narrowly shaped fruits. The crosses involving two plants with heart-shaped and the narrowly shaped fruits respectively, result in a progeny having heart-shaped fruits. The F2 progeny show 15 heart shaped and one narrow shaped plant.
References:
[1] Principles Of Genetics 7/E, By Tamarin
[2] Essential Genetics: A Genomics Perspective, Hartl, Elizabeth W. Jones
[3] Competition Science Vision
[4] Principles of genetics, 8th ed, Gardner, m. J. Simmons, d. P. Snustad
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