"By definition, eating disorders are complex traits. That means that their inheritance pattern in families does not follow traditional Mendelian patterns, and that they are influenced by multiple genetic and environmental factors of small to moderate effect. There is not one gene for anorexia nervosa or one gene for bulimia nervosa. More likely there are a number of genes that code for proteins that influence traits that index vulnerability to these disorders. Complicating the risk picture even further, these genes exist in concert with other genetic factors that may confer protection against eating disorders, along with main effects of risk and protective environments, as well as gene x environment interplay as we discuss in the following section."