In epistasis, one gene affects the expression of another. Perhaps the most challenging problem for genetics is the explanation of complex phenotypes that are caused by many interacting genes and the environment.
learning outcomes
You should be able to:
Analyze genetic data involving epistasis.
Compare and contrast penetrance and expressivity.
Compare and contrast quantitative and qualitative variation.
Explain the difference between penetrance and expressivity.
Penetrance is the proportion of individuals with a certain genotype that actually show the phenotype associated with that genotype; that is, what proportion of pea plants with “Tall” genotype are tall? Expressivity is the extent to which a genotype actually shows its associated phenotype; that is, are all the pea plants with the “Tall” genotype equally tall, or is there a gradation of height?
How is quantitative variation different from qualitative variation?
Qualitative variation involves phenotypes, such as colors and textures, that cannot be easy measured; for example, green and yellow peas. Quantitative variations are phenotypes that can be measured; for example, tall and short pea plants.
If the dominant allele A is necessary for hearing in humans, and another allele, B, located on a different chromosome, results in deafness no matter what other genes are present, what percentage of the offspring of a parental cross of aaBb × Aabb will be deaf?
75%
In the next section we’ll see how the discovery that genes occupy specific positions on chromosomes enabled Mendel’s successors to provide a physical explanation for his model of inheritance, and to provide an explanation for those cases where Mendel’s second law does not apply.