in biology, the concept that the biological effects of radiations (radiation) such as X rays result from ionization (i.e., the formation of electrically charged particles) by individual quanta, or photons, of radiation that are absorbed at sensitive points (targets) in a cell. It is supposed that to produce a given effect there must be one or more hits on a target. Ionization of a target molecule of genetic material produces a direct effect on the constitution of the cell, which may be passed on to the cell's progeny. The theory has been of value in providing a quantitative basis for evaluating many of the biological effects of radiations, particularly in the field of genetics.