ABSTRACT
The predominant environmental factors in the causation of cancer include chemicals, ionizing and ultraviolet radiation, as well as specific infectious agents, predominantly viruses. Although the majority of these agents may have as a principal component of their etiological mechanism some interaction with and/or alteration of the cellular genome, neoplastic disease resulting from the action of such agents is not generally thought of as hereditary or genetic disease. The term hereditary or genetic disease usually connotes an abnormality transmitted through the germline from parent to offspring. In this sense it is reasonable to state that, in considering all cases of human neoplasia, most cancers are not the direct result of heredity but are acquired through an interaction of the host with the environment. However, the interaction of the environment with the genetic composition of the host, either directly or indirectly through the regulation of the expression of the host genome, is important in the development of all human and animal neoplasms. Therefore, although relatively few human neoplasms exhibit a clearly defined Mendelian pattern of heredity, polygenic and multifactorial inheritance may play a significant role in increasing the risk of cancer for a large number of humans. This discussion centers largely on hereditary factors in human cancer, because there is a larger body of knowledge on this subject in this species; however, germline genetic factors in the development of cancer in lower animals are also considered.
