When the Human Genome Project is completed, approximately 4,000 gene disorders will have been identified. In other disorders both of the paired genes must be abnormal for the disease to be expressed. Sickle cell anemia and Tay-Sachs disease are examples of this type of disorder. Breast cancer, diabetes, heart disease, lung cancer, mental illness, alcoholism, and obesity are all multifactorial genetic disorders. A woman who carries a mutation of the BRCA1 or BRCA2 gene has a 50-85 percent chance of developing breast cancer, but whether she actually will is influenced by hormonal, dietary, and other genetic factors.
Mutations can also arise spontaneously, relating to aging, behavior, and the environment.
The description of our genetic map will allow the testing of our DNA for errors. This information could predict whether an individual will develop a disease or whether he or she is only at higher risk and could modify that risk. During the next decade DNA testing for both specific gene disorders and multifactorial genetic predisposition will become commonplace. Thousands of new tests will become available, and many will be expensive. Today there is little or no regulation of the quality of genetic testing labs and testing standards are few.