Genetic interactions: enhancement and suppression - AndersenLab/Genetic-Analysis GitHub Wiki
Incomplete Penetrance vs. Expressivity
Not all individuals with a given genotype will have the same phenotype, nor will it necessarily be expressed to the same degree. These two phenomenon are referred to as incomplete penetrance and variable expressivity.
In a trait that is incompletely penetrant, not every individual with a given genotype will express the phenotype. For example, many genetic variants contribute to the risk of someone developing a disease such as cancer or autoimmune disorders. However, not all individuals will contract the disease.
Similarly, the severity of a phenotype or disease can differ among people with the same genotype. For example, individuals with cystic fibrosis commonly inherit two copies of the ΔF508 mutation. However, the severity of the disease can vary considerably, and is modulated by many additional environmental and genetic factors.
Determining Incomplete Penetrance
How is it possible to determine when incomplete penetrance is occurring? In clonal populations of organisms (such a C. elegans) it is very straightforward. If the trait being examined is observed to be incompletely penetrant, then individual worms can be isolated and allowed to self-fertilize and produce offspring. You can then examine the resulting phenotypes and observe whether the trait is again observed with a similar frequency.
Suppressors and Enhancers
Suppressors - make the mutant phenotype better. The normal function of the gene is to act in opposition to a suppressor.
Enhancer - make mutant phenotypes worse. The normal function of the enhancer gene acts with the same effect as the original gene.
Further Resources
- Penetrance and Expressivity - A good overview.