Phenylalanine is present in many, but not all, foods, and it takes knowledge and vigilance to avoid them all. Because the worst aspects of PKU can be avoided by avoiding foods with phenylalanine, most states require screening for PKU at birth. The majority of diseases, however, result from a more complex interaction between genetics and environment. Several genes are often involved, and there are several environmental factors that contribute to these diseases. We showed asthma as an example in the figure above.
Cancer is a special case.
It arises from a combination of many genetic defects, some caused by the environment and some potentially inherited. The number of inherited defects determines a person's susceptibility to cancer. We'll talk more about this later. Also, fluoride, pesticides, minerals. Also includes infection-related factors, such as sanitation and proximity to people, animals, or insects.