Penetrance is defined as the percentage of individuals with a given genotype, which exhibit the phenotype associated with this genotype. For instance, all individuals with the genotypes AA or Aa, may not express the dominant phenotype, due to -either the presence of modifiers, suppressors, epistatic genes, etc. or due to the modifying effect of the environment. If only 80% of individuals with AA or Aa express dominant phenotype, it will be described as 80% penetrance of gene (or allele) 'A' Expressivity on the other hand, describes the degree or extent to which a given genotype is expressed phenotypically in a particular individual. This may also be due to lack of full expression due to rest of the genome and the environment. For instance, the red eye colour in Drosophila flies may show different shades of colour (within the norm of reaction) suggesting variable expression of wild allele ‘W’- in WW or Ww flies. The distinction between penetrance and expressivity is illustrated in Figure 3.3, and variable expression of coat colour in dogs is shown in Figure 3.4.