While some traits are determined by a single gene, many others are dictated by a complex array of factors. The environment often plays a role, to greater or lesser degrees, changing where, when, and the degree to which a gene or genes are expressed. Many traits, including most traits related to complex health and disease phenotypes and behaviors, are influenced by more than one gene. These are said to be polygenic traits. When a trait is the result of a combination of many genes plus environmental influences, it is said to be the result of multifactorial causation. Note that inheritance may be modified by "polygenic" (because genes can be inherited) but multifactorial should not be used in this way (because environmental conditions are not heritable, even though their effects may be, in some cases).