A hereditary tendency toward a condition. Many modern health problems appear to have hereditary components, from certain forms of cancer and heart disease to Alzheimer’s disease and Parkinson’s disease. Generally a number of other factors, such as nutrition or exposure to environmental triggers, must converge to allow a genetically predisposed condition or disease to develop.
A genetic predisposition for a health condition does not make development of the condition inevitable. Having a genetic predisposition differs from having a genetic defect, in which the condition will occur regardless of other factors. A defective, or mutated, gene causes the neurodegenerative condition huntington’s disease, for example, and every person who has the mutated gene is certain to develop the disease by middle age. Having mutated genes that are linked to conditions such as Parkinson’s and Alzheimer’s, however, does not alone determine whether the disease will manifest itself.