A change in the chemical constitution of the DNA in the chromosomes of an organism: the changes are normally restricted to individual genes, but occasionally involve serious alteration to whole chromosomes. When a mutation occurs in gametes or gametocytes an inherited change may be produced in the characteristics of the organisms that develop from them. Mutation is one of the ways in which genetic variation is produced in organisms. A somatic mutation is one that occurs to a body cell, and is consequently past on to all the cells derived from it by mitosis. Natural mutations, at this stage of biological evolution, when they occur in the cells of higher animals, almost always produce deleterious characteristics. Both natural and artificial mutations can be brought about by ionizing radiation (hence the genetic and carcinogenic dangers of nuclear weapons) and by certain chemical substances called mutagens.