Many of our inherited characteristics are multigenic or even multifactorial in nature. In other words they depend on the interaction between multiple genes or between genes and external factors. This means that a disease that is essentially genetic can also be triggered by environmental factors.
If multiple elements are responsible for a characteristic or disorder this is known as multifactorial inheritance. Physical build and intelligence are examples of 'normal' multifactorial characteristics.
Known multifactorial conditions include spina bifida, cleft lip and palate, club foot, congenital cardiac disorders and Crohn's disease.