Polygenic disease (or polygenic disorder) results from the effects of the combined action or interaction of multiple genes. More than one gene is associated with a polygenic disease or disorder. Because of the complexity of genetic interactions, it does not follow a pattern the same way a monogenic disease does. In humans, though, polygenic conditions occur more frequently than monogenic diseases.
Apart from the multiple genes involved, non-genetic factors may also play a role in the manifestation of the disease. For instance, type 2 diabetes is a disease characterized by the presence of excessive glucose levels circulating in blood and symptoms such as polyphagia, polyuria, and polydipsia. It is a polygenic disease having involved with more than one gene, i.e. more than 36 genes (e.g. TCF7L2 allele). These genes increase the risk of developing diabetes. Many of these genes are linked to beta cell functions. Other factors associated with the contraction of diabetes include non-genetic factors, e.g. lifestyle and diet.