Penetrance: defect is present in all cells but not always expressed, depends on the individual but difficult or impossible to predict who will be affected, so it is more of a lottery.

Where penetrance is incomplete, the disorder may or may not be expressed. The frequency with which the gene defect occurs in individuals is usually expressed as a percentage. Hereditary haemochromatosis (=a disease in which iron accumulates) is one example of a disorder with variable penetrance.


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