There are various ways in which genes at different loci can interact with each other. The ability of a gene at one locus to effect the expression of a gene at another locus is termed epistasis. There are various forms of epistasis. They are as follows:

Dominant Epistasis -
When the dominant allele at one locus (homozygous dominant or heterozygous) prevents expression of one or more alleles at another locus (homozygous or heterozygous).

Recessive Epistasis -
When the recessive allele at one locus (in the homozygous recessive condition) prevents expression of alleles at another locus (homozygous or heterozygous).

Duplicate Genes with Cumulative Effect -
When the dominant gene at one locus (homozygous dominant or heterozygous) produces the same phenotype as a dominant gene at another locus. However, when both loci are dominant (homozygous dominant or heterozygous) there is a cumulative effect that produces a distinctive phenotype.

Duplicate Dominant Genes -
When the dominant gene at two different loci (homozygous dominant or heterozygous) each produce the same phenotype without a cumulative effect.

Duplicate Recessive Genes -
When the recessive genes at two different loci (both homozygous recessive) produce the same phenotype without a cumulative effect.

Dominant and Recessive Interaction -
When the dominant gene at one locus (homozygous dominant or heterozygous) produces the same phenotype as a recessive gene at another locus (homozygous recessive).

Genes that act together to produce differences in the degree between phenotypes are termed polygenes. This form of inheritance is different than the classical Mendelian type gene or major gene. As previously mentioned, not all traits are controlled by a single pair of genes - a trait can be controlled by numerous genes, perhaps up to 100 or more.

There are various ways that alleles at the same loci can act with each other to produce a particular phenotype. Allelic interactions at the same loci are categorized as follows:

Complete Dominance -
The homozygous recessive alleles produce one phenotype and the homozygous dominant alleles and heterozygous recessive produce another.

Partial or Incomplete Dominance -
The heterozygous condition produces a phenotype intermediate between the homozygous conditions.

Overdominance -
The phenotypic expression of the heterozygous condition exceeds the phenotype of the homozygous dominant condition.

Additive Genes -
In a model situation, one allele contributes nothing to the phenotype, the other allele contributes by a factor of one in the heterozygous condition and by a factor two in the homozygous condition. In other words, the phenotype of heterozygote is exactly intermediate between either homozygote, a special case of partial dominance.

Keep in mind that phenotypes controlled by genes at different loci can interact to create the appearance of a particular case mentioned above (e.g.,duplicate dominant gene interaction).

Two terms used to describe how often a gene displays itself are penetrance and expressivity. Penetrance is defined as the percentage of individuals with a particular gene combination (genotype) that exhibit the corresponding character to any degree. If the dominant gene in the heterozygous condition was not expressed all the time then the penetrance is some percentage less than 100%. Most dominant genes have a penetrance of 100%. Expessivity is the degree of effect produced by a particular penetrant genotype. The Pied trait is a good example of expressivity - ranging from several white patches in some birds to completely white in others.

This presentation is a general overview of a very complicated topic. I feel that the information presented is important because the more breeders understand about genetics the better we are in obtaining and preserving new viable mutations. If you have a true interest in learning more about genetics, I would strongly recommend purchasing the Shaum’s Outline of Genetics from McGraw-Hill. It is an outstanding text from which to study and learn genetic principles.

In the future, I hope to expand this presentation into a more detailed genetic description of cockatiel plumage mutations. This will require statistical breeding data, detailed phenotypic descriptions, photographs, etc. and a lot more research on my part. Any information that could be provided to help in this endeavor would be appreciated.

References