In complex disease, both genetic factors and environmental exposures contribute to the development of disease. If the genetic predisposition is strong or highly penetrant, less environmental exposures are needed to potentially develop disease (A). If the genetic predisposition is mild, or has low penetrance, there may need to be more or stronger environmental exposures for the disease state to occur (C). For all of these examples, however, it is possible that the disease never develops within an individual's lifetime.
Penetrance refers to the strength of a gene to cause disease. Some genetic factors are stronger than others in causing disease. The "stronger" the gene, the higher the penetrance.