Definition for Variable expressivity
From Biology Forums Dictionary
Variable expressivity is exactly as it sounds; persons with the same gene or changed gene in their genotypes can have phenotypes that vary. An example of variable expressivity is with a genetic condition called cystic fibrosis that requires two inherited mutations in the CFTR gene to have the associated disease phenotype. Cystic fibrosis is a common genetic disease in the Caucasian/Northern European populations in which there are over 1000 known mutations in the CFTR gene, but the different mutations (genotypes) do not clearly correspond with how significantly the person is affected by cystic fibrosis (phenotype). Essentially, persons with cystic fibrosis who are and who are not related can all have inherited the same exact mutations, but can all present clinically with very different symptoms and degrees in severity of cystic fibrosis.