Individuals that carry a same genotype do not always manifest in an identical manner the correspondent phenotype. These manifestations may differ in intensity, from one individual to another, or even the phenotype may not manifest in some percentage of carriers.
Gene penetrance is the percentage of phenotypical manifestation of a gene in a given population of carrier individuals (same genotype). Gene expressivity is the degree (intensity) of the phenotypical manifestation of a gene in each individual or group of individuals that carry the gene (same genotype). The gene penetrance and the gene expressiveness may be influenced by the environment.