Describe the difference between incomplete penetrance and variable expressivity.

In incomplete penetrance, a genotype is not expressed by every organism in which it is present. In variable expressivity, the organisms that share a genotype express the corresponding phenotype to different degrees. Both are explained by genetic or nongenetic interactions that modify or prevent the consistent expression of a genotype.

Three kinds of interactions may be responsible: (1) other genes that act in ways that modify the expression of the mutant allele, (2) environmental or developmental (i.e., nongenetic) factors that interact with the mutant allele to modify its expression, and (3) some combination of other genes and environmental factors interacting to modify expression of the mutation. In inbred laboratory strains of model genetic organisms, variation in genetic factors can be eliminated experimentally to allow separation of gene-gene and gene-environment variability, something that cannot be done in organisms such as humans.