Most diseases do not follow simple patterns of inheritance.
Many factors influence a gene's ability to build proteins. For one thing, different mutations in the same gene can produce a wide range of effects. In cystic fibrosis, for example, the gene that controls mucus production can have more than 300 different mutations; some cause severe symptoms; some, mild symptoms; and some, no symptoms at all.
![Disease Inheritance Is Complex](https://webarchive.library.unt.edu/web/20081021101936im_/http://www.cancer.gov/images/Documents/2bb8687b-eb2b-4e62-ba30-a0572dee414e/cancer18.jpg)
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