Principles of Genetics and Breeding

3.1.3 Epistasis

• A gene or locus which suppressed or masked the action of a gene at another locus was termed suppressor or epistatic gene.
• The gene or locus which was suppressed by an epistatic gene was called hypostatic gene.
• Dominance is the masking effect which one allele has upon the expression of another allele at the same locus (i.e, intra-genic or inter allelic gene suppression).
• Epistasis involves inter-genic suppression or the masking effect which one gene locus has upon the expression of another. Thus, epistasis refers to variation resulting from the interaction of alleles at different loci.
• Epistatic interaction between two loci produces variation on the 9:3:3:1 F₂ phenotypic ratio that occur when there are two dominant genes which produce different phenotypes.
• When there is epistasis, the number of F₂ phenotypes is usually reduced from four to either two or three,depending on the type of epistasis.

Table 5.1. Autosomal phenotypes controlled by epistasis

Species	Genes	Phenotypes
Common carp	S, N	Scale pattern
Gold fish	B1 , B2 M, S DP1 , DP2	Orange body colour Albino Depigmentation of melanophores

Body colour in many tropical fish is controlled by epistatic interactions between or among two or more loci. A set of qualitative phenotypes may be controlled by more than two genes.

• Body colour in the Siamese fighting fish is an example of a set of phenotypes that is controlled by the epistatic interaction among four genes.
• Working with these phenotypes is far more complicated because of the number of genes involved. Fortunately,in food fish, no qualitative phenotype controlled by more than two genes has been discovered.
• Scale pattern in common carp is the most important phenotype controlled by epistasis. An understanding of the inheritance of scale pattern in common carp is of great important because common carp with a reduced scale pattern has a higher market price in Europe. Common wild type scale pattern (scaled) is more desired in Asia. Since common carp forms the world’s most important cultured food fish, the genetic technique to produce the desired scale phenotype can be of tremendous importance. In growth rate, scaled carp are better than the scattered ones and the linear ones are better than the nude ones.
• Scale pattern in common carp is controlled by the S and the N genes. They produce the phenotypes through a type of dominant epistasis where the N gene is the epistatic locus, but it is a dominant lethal epistatic gene (it exhibits incomplete dominance and is lethal with homozygous state).
• The S gene controls the scaliness, and the N gene modifies the pattern. The S allele is completely dominant over the s allele. In the presence of the gene N the action of the gene S is stronger. S allele is the common wild type, s allele produces reduced number of scales, and those that remain are greatly enlarged (mirror).
• A single N allele changes scaled carp into line carp (scales limited to the dorsal and ventral margins and the lateral line) and changes mirror common carp into leather or nude common carp (no or virtually no scales).
• The N allele is lethal in the homozygous dominant state. The N allele has no effect on scale pattern.

So far we have assumed that scaling pattern loci in carp only affect scale density and pattern. However,allelic variation at a single locus may have effects on more than one biochemical pathway and this can have subtle effects on other phenotypic characters. This is called pleiotropy and there are a number of performance based pleiotropic effects on mirror, line and leather carp. The wild-type scaled carp generally grow, survive and resist disease better than do mirror, line or leather carp.

Figure 5.1. Inheritance of scale pattern in common carp.Scale pattern is determined by the epistatic interaction between the S and N genes.