Principles of Genetics and Breeding

3.1.5 Dominance

Complete dominant gene action

• Complete dominant gene action occurs when the dominant allele is so strong that it produces its phenotype, regardless of the genotype.
• Only a single dominant and heterozygous genotypes produce the dominant phenotype; thus, the phenotypes produced by these genotypes are identical.
• The recessive allele can produce the recessive phenotype only when a fish is homozygous recessive. Consequently, with this mode of inheritance, there are three genotypes but only two phenotypes as described by Mendel.
• An example of a phenotype controlled by a single autosomal gene with complete dominance is albino and normally pigmented channel catfish (Ictalurus punctatus). Normal pigmentation is the dominant trait and is produced by the ++ and +a genotypes. Albinism is the recessive trait and is produced by the aa genotype.
• Table 5.3 lists some qualitative phenotypes in important cultured food fishes and ornamental fishes that are produced by single autosomal genes with complete dominance.

Table 5.3. Phenotypes controlled by singleautosomal genes with complete dominant gene action.

Species	Allele		Dominant phenotype	Recessive phenotype
Channel catfish	+	a	Normal pigmentation	Albino
Common carp	B G Gr	b g gr	Normal pigmentation Normal pigmentation Normal pigmentation	Blue Gold Grey
Grass carp	A	a	Normal pigmentation	Albino
Rainbow trout	G B	g b	Normal pigmentation gold Normal pigmentation	Albino Metallic blue
Goldfish	B D +	b d +ne	Orange – red Normal – eyes Normal scales	Blue Telescope eyes Nacreous – like
Guppy	Sn B G	Sc b g	Normal spine Greenish grey Greenish grey	Curved spine Blond soft yellow body colour Gold
Medaka	F W	f w	Normal Normal spine	Fused vertebrae Curved spine
Platyfish	St	st	Stippled	Unstippled
Nile tilapia	BL b	Bl b	Normal pigmentation Normal pigmentation	Blond light coloured (pink )

• For example, the gold colour variant in common carp (Cyprinus carpio), is controlled at a single locus by two alleles, a wild-type allele (G) and a variant gold producing allele (g). The gold phenotype is only expressed in the fish when the g allele is present in the homozygous condition (i.e. the genotype is gg),while the other two genotypes (GG and Gg) will always have wild-type colouration. The GG is present either in the homozygous or heterozygous condition and is described as dominant. Conversely, the g allele is described as recessive because its phenotypes is not expressed in the heterozygous condition.
• If the gold variant is of high market value, the breeder can produce them in large numbers in the following method.
• By mating a gg male with a gg female to ensure gg offspring; however, in wild population, where are likely to be only occasional individuals expressing the gold coloration. So, to begin with, if we mate our novel gold-colored fish to a wild-type fish, usually no gold offspring would appear in the F₁generation because the gg allele being present in both the father and the mother, no gg genotypes–and therefore no gold phenotypes–will be present among the F₁ offspring.
• However, the F₁ offspring will be heterozygotes with one copy of the G allele and if these are mated together then this will produce gg in F₂ generation.
• “Blue”colour in carps of domesticated varieties is inherited as a simple recessive trait. This blue color results from the under development of guanine crystals associated with the reduction of guanophores in the skin of the carp. This condition is known as alampia.