Penetrance refers to the proportion of people with a particular genetic alteration that exhibits signs and symptoms of an inherited disease. When some people with a variant do not develop any of the features of the disorder, they are said to have reduced (or incomplete) penetrance.
Some genetic disorders differ a little, but in most cases the affected individuals show different signs and symptoms. Variable expressivity refers to the range of signs and symptoms that can appear in different people with the same genetic condition.
When there is variable expressivity, the trait may vary in clinical expression from mild to severe. For example, neurofibromatosis type 1 can be mild with only café-au-lait spots or severe with neurofibromas and brain tumors.