Principles of Genetics and Breeding

3.1.5.3 Additive gene action

When there are more than 2 alleles, they all combine together and express a phenotype. When the mode of gene action is additive, neither allele is dominant, and each allele always produces its phenotype in a unidirectional step-wise manner. When this occurs, a gene with additive gene action produces a unique phenotype for each genotype:

• Only one additive gene has been discovered in fish; it controls golden, palomino, and normal body colours in rainbow trout, Oncorhynchus mykiss.
• The G gene in rainbow trout is an example of a gene with additive gene action. The G gene produces the golden, palomino, and normally pigmented body colour types. The mating between two heterozygotes will produce progeny with 1:2:1 genotypic and phenotypic ratios.

Genotype	Phenotype
G1G1	Golden (red)
G1G	Palomino (dark yellow)
GG	Normally pigmented

• Additive interactions occur when two or more loci affect the same character. When phenotypes are controlled by additive gene action, there is no dominant or recessive allele.
• Their effects are additive. Both alleles contribute equally to the production of the phenotypes, so the heterozygous genotype produces a phenotype that is intermediate between those produced by the two homozygous genotypes.
• Three phenotypes of the sticklebacks are controlled by additive action of 2 pairs of genes A and a, B and b.

Phenotypes Strong Intermediate Weak

Genotypes AABB AaBB AABb aaBBAaBb AAbb aaBb Aabb aabb