Multifactorial disorders involve variations in multiple genes, often coupled with environmental causes. Polygenic disorder caused by the combined action of more than one gene. In an age when the majority of monogenic human disease genes have been identified, one of the challenges for the coming generation of human geneticists will be to resolve complex polygenic and multifactorial disorders, as human diseases are often caused by a multitude of genetic and environmental factors acting in concert. A number of single gene disorders are relatively rare when compared to multifactorial and polygenic diseases. Congenital malformations that are thought to have complex multiple interacting causes include congenital heart defects, neural tube defects, pyloric stenosis, cleft palate and congenital hip dysplasia. The term polygenic can have different meanings, including genetic effects that arise from the interaction of multiple genes. Multifactorial inheritance describes a trait whose manifestations are determined by two or more genes, accompanied by environmental factors.