Gene Interactions

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:

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:

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.

Closing Remarks

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.


Cooke and Buckley, 1987, Avian Genetics A Population and Ecological Approach, Academic Press Inc., Orlando, Fl

Gardner, Eldon, 5th edition, 1975, Principles of Genetics, John Wiley & Sons, Inc., NY, NY.

Keeton, William, 2nd edition, 1972, Biological Sciences, W. W. Norton & Co, Inc., NY, NY

Stansfield, William, 3rd edition, 1991, Shaum’s Outline of Genetics, McGraw-Hill , NY, NY

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