Principles of Genetics and Breeding

3.1.3.1 Dominant and recessive Epistasis

• Dominant epistasis occurs when a dominant allele at one locus (the epistatic locus) produces a particular phenotype, regardless of the genotype at the second locus. The second gene can express its phenotype only when the epistatic locus is homozygous recessive.
• For example, out of two genes, the dominant allele (e.g., A) of one gene masked the activity of alleles of another gene (e.g., B) and expressed phenotypically, then A gene locus is said to be epistatic to the B gene locus. Because, the dominant allele A can express itself only in the presence of either B or b allele, therefore, such type of epistasis is termed as dominant epistasis.
• The alleles of hypostatic locus or gene B will be able to express themselves phenotypically only when gene locus A may contain two recessive alleles (aa).
• Thus, the genotype AABB or Aa Bb and AA bb or Aa bb produce the same phenotype whereas the genotype aa BB or aa Bb and aa bb produce two additional phenotypes. The dominant epistasis modify the classical ratio of 9:3:3:1 in to 12:3:1.
• In goldfish, two loci are involved simultaneously in the expression of albinism. Albinism in goldfish is an example of a phenotype that is controlled by dominant epistasis by the M and the S genes . M gene is the epistatic locus. A single dominant M allele produces dark goldfish. When the M locus is homozygous recessive (mm), the S locus can produce either light (SS, Ss) or albino (ss) goldfish.Consequently, albinos can only be produced when a goldfish is homozygous recessive at both loci (mm, ss). and
• Recessive epistasis occurs when the recessive alleles of one gene locus (aa - the epistasisl ocus) suppress the phenotypic expression of the alleles of another gene (BB, Bb or bb alleles). This type of epistasis is called recessive epistasis. The alleles of B-locus express themselves only when epistatic locus A has dominant allele (e.g., AA or Aa). Recessive epistasis produces a 9:3:4 F₂ phenotypic ratio. E.g. Eye colour in Mexican cavecharacins. Black, brown and pink eye colour are controlled by the ab and bw genes. ab locus is the epistatic locus–ab ab produces pink eyes, regardless of the bw genotypes. A single dominant ab allele (+) allows the bw genotype to produce either brown or black eyes. F₂ phenotype ratio for the mating of two heterozygous ab +, bw + black eyed mexican cave characins are 9 Black : 3 Brown : 4 Pink. phenotype ratio.Table 5.2. Phenotypes controlled by the different types of epistatic interaction of two genes in fish

Species	Phenotype	Type of epistasis	F2 phenotypic ratio
Common carp	Scale pattern	Dominant epistasis	12:3:1
Chinook salmon	Flesh colour	Duplicate recessive gene interaction	9:7
Gold Fish	Albinism	Dominant epistasis	12:3:1
Mexican cave characins	Eye colour	Recessive epistasis	9:3:4
Sumatran tiger barb	Trunk striping	Duplicate genes with cumulative effects	9:6:1