Common features of complex human diseases (for example Alzheimer’s disease, Crohn’s disease and cardiovascular diseases) are that they present mostly a relatively mild phenotype, are slowly progressive and chronic in nature.3 The patho-physiology of complex diseases is characterized by various biological pathways, leading to similar clinical phenomena. Importantly, complex diseases are associated with variations in multiple genes, each of which has a small overall contribution and relative risk for the disease process.4 Similarly to other complex diseases, we estimate that, for periodontitis, at least 10 and possibly as many as 20 disease-modifying genes may be involved. However, it is important to realize that the number and types of disease-modifying genes for the same condition may not be equal for different forms of periodontitis and different ethnic populations; their effect is also influenced by environmental factors (gene–environment
Common features of complex human diseases (for example Alzheimer’s disease, Crohn’s disease and cardiovascular diseases) are that they present mostly a relatively mild phenotype, are slowly progressive and chronic in nature.3 The patho-physiology of complex diseases is characterized by various biological pathways, leading to similar clinical phenomena. Importantly, complex diseases are associated with variations in multiple genes, each of which has a small overall contribution and relative risk for the disease process.4 Similarly to other complex diseases, we estimate that, for periodontitis, at least 10 and possibly as many as 20 disease-modifying genes may be involved. However, it is important to realize that the number and types of disease-modifying genes for the same condition may not be equal for different forms of periodontitis and different ethnic populations; their effect is also influenced by environmental factors (gene–environment