He began using fruit flies about 1907 and found instead that mutations were not new species and generally resulted in a loss of function and that they were generally recessive. It is only expressed when both genes are knocked out. Other geneticists noted other abnormalities and called them mutations too. As genetic analysis revealed their more complex nature as losses or gains of a single chromosome or as deletions or additions of chunks of chromosomes, the term mutations meant many things to many persons. Geneticist Herman Muller clarified the confusion by defining mutation as a change in the individual gene.
Most mutations arise spontaneously.
They can also be induced by x-rays, ultraviolet light, and a variety of chemicals. These share in common a capacity to damage DNA, the genetic material that composes our genes. Hydrogen peroxide and formaldehyde are chemical mutagens most people would recognize. Less familiar are caustic chemicals used by industry called alkylating agents. The chemicals I mentioned actually alter a purine or a pyrimidine in DNA by chemically modifying it.