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