In the study of chronic diseases, both environmental and genetic factors can be influential. In highly common diseases, such as coronary heart disease, genetic effects may be more influential in determining the age of onset of the disease than in determining whether or not one gets the disease. When sufficient information is available, family studies can help localize possible disease genes on the human chromosome through genetic linkage analysis, and familial aggregation of disease can help separate the effects of inheritance, environment and lifestyle on the risk of disease.