The completion of the Human Genome Project opens the door to a better understanding of the basic causes of many human diseases. A great many diseases have a genetic basis. There are three types of genetic diseases:

• Monogenic
• Oligogenic
• Polygenic

Monogenic diseases are those in which defects in a single gene produce disease. Often these disease are severe and appear early in life. Examples are familial hypercholesterolemia and cystic fibrosis. In fact there are hundreds of monogenic disorders; but fortunately for the population as a whole, they are relatively rare. In a sense, these are pure genetic diseases. They do not require any environmental factors to elicit them. Although nutrition is not involved in the causation of monogenic diseases, these diseases can have implications for nutrition. They reveal the effects of particular proteins or enzymes that also are influenced by nutritional factors. Thus, we have learned a great deal about human metabolism from monogenic diseases.

Oligogenic diseases are conditions produced by the combination of two, three, or four defective genes. Often a defect in one gene is not enough to elicit a full-blown disease; but when it occurs in the presence of other moderate defects, a disease becomes clinically manifest. It is the expectation of human geneticists that many chronic diseases can be explained by the combination of defects in a few genes. These are called major genes. If so, an understanding of the causation of complex genetic diseases should be possible.

A third category of genetic disorder is polygenic disease. According to the polygenic hypothesis, many mild defects in genes conspire to produce some chronic diseases. To date the full genetic basis of polygenic diseases has not been worked out; multiple interacting defects are too complex to understand at present. Nonetheless some human geneticists believe that polygenic factors contribute importantly to chronic diseases such as coronary heart disease, cancer, diabetes, and osteoporosis. Whether these diseases will ultimately turn out to be oligogenic or polygenic in origin remains to be determined.

The Center for Human Nutrition is interested in human genetics because of the fact that both genes and nutrients modify the same metabolic processes. What is learned about genetic control of metabolism has implications for human nutrition and vice versa. The essential paradigm of the Human Genetics Project is shown in the figure below. Dr. Jonathan Cohen heads the Human Genetics Project of the Center for Human Nutrition. Dr. Cohen has a part time appointment in the Center for Human Genetics, headed by Dr. Helen Hobbs. With this connection Dr. Cohen is in a position to bring the power of genetic research at UT Southwestern and the Human Genome Project into the research endeavors of the Center for Human Nutrition.