Complex diseases do not have a single cause; it is the interaction between your genes, environment and lifestyle that ultimately cause complex diseases to occur. Examples of complex diseases are cancer, heart disease, and diabetes.
To better understand how these factors work together, we can compare the risk of developing a complex disease to the risk of having a car accident.
The risk of having a car accident is influenced by several factors:
- A car that is not working properly
- A driver that is distracted or impaired
- Weather related hazards like rain, snow and ice
- Roads that need repair
With the addition of each factor, the risk for an accident increases. Some, but not all of the risk factors can be controlled by the driver. Similarly, the risk of developing a complex disease changes based on the combination of our genes, our lifestyle, and our environment. Like the driver in this example, we can control some of the risk factors (lifestyle factors) but not others (genetic factors).
How Much is Genetic Versus Non-Genetic?
While it is difficult to determine exactly how much influence any single risk factor possesses, we can estimate the percent of a disease or trait that is due to genetic factors (heritability) and the percent of a disease or trait that is due to non-genetic factors, like lifestyle and environment.
Approximately 21% of the risk for melanoma is believed to be due to genetic factors and 79% is believed to be due to non-genetic factors (such as sun exposure).
For type 1 diabetes, it is estimated that genetic factors make up about 88% of the risk, while the remaining 12% of the risk is due to non-genetic factors (such as virus exposure).
In both of these examples, the contribution of genetic factors versus non-genetic factors is an estimate. These estimates are based on studies of families and twins. Family and twin studies look at how often a disease occurs within a family or how often a disease occurs in both twins.
Genetic Risk Factors
Genetic variants contribute to the risk of developing common and complex diseases, such as cancer and diabetes. When dealing with complex diseases there are many variants that contribute to disease risk. Each of these genetic risk factors contribute to your disease risk in a different way. In some cases, the contribution to your disease risk will be small, while in other cases having a specific variant can have a large impact on your disease risk. Your overall risk based on your genes depends upon how many genetic risk factors you have and how they interact with one another.
For example, variants in more than 28 genes have been associated with type 2 diabetes. It is not known exactly how many of these variants a person must have in order to develop type 2 diabetes. It is also not completely understood how non-genetic risk factors, like body mass index, interact with genetic risk factors to produce disease.
Non-Genetic Factors: Environment and Lifestyle
In addition to genetic variants, non-genetic variants contribute to the risk for developing common diseases. These non-genetic risk factors can be grouped into two different categories: environmental and lifestyle risk factors.
Examples of environmental risk factors are sun exposure, air quality, and job-related hazards involving exposure to chemicals, radiation, and carcinogens.
Lifestyle can also contribute to our disease risk and our overall health. Lifestyle risk factors like diet, exercise, smoking, and alcohol consumption can play a large role in determining whether or not someone develops a disease like heart disease or type 2 diabetes. In many cases, lifestyle risk factors play a bigger role in the development of a common, complex disease than genetic risk factors.
Family history is also an important tool for determining the risk of developing a disease. Family history is often the best predictor of disease because it ties together both genetic and non-genetic risk factors. Family members share genes and often share similar environments and lifestyle habits.
Taking Control of Your Health
Having a genetic variant that is associated with a disease does not mean that you will definitely get that disease. It means that you are at higher risk of developing that disease than a person that does not have that genetic variant. While genetic risk factors cannot be changed, knowing about non-genetic factors such as lifestyle and environmental risk factors can help you identify those risk factors in your life that you can take control over and change.
For example, smokers have double the risk of developing age-related macular degeneration compared to someone who has never smoked. By quitting, you can reduce your risk by 60%.