_Mutagen-Induced Gene Mutation._ The narrator of this film is Hermann J. Muller, who won a Nobel Prize in 1946 for his work in the field of genetics. The measurement of X-ray dose in roentgens and the dose required to double the spontaneous mutation rate in _Drosophila_ and mice are discussed. The magnitude and meaning of permissible doses of high-energy radiation are discussed. Other mutagenic agents (ultraviolet light and chemical substances) are discussed, concluding with comments on the importance of gene mutation in the present and future. _Selection, Genetic Death and Genetic Radiation Damage._ The narrator of this film is Theodosius Dobzhansky, the coauthor of this booklet. Genetic death is discussed in detail, as are examples of how genetic loads are changed subsequent to radiation exposure. While it is generally agreed that the great majority of mutants are harmful when homozygous, more evidence is needed about the beneficial and detrimental effects of mutants when heterozygous. In the case of sickle cell anemia, heterozygotes are adaptively superior to normal homozygotes. This makes for balanced polymorphism, by which a gene is retained in the population despite its lethality when homozygous because of the advantage it confers when heterozygous. _Gene Structure and Gene Action._ The lecturer of this film is G. W. Beadle of Cornell University. The Watson-Crick structure of DNA is discussed in terms of mutation. Several tests of the chain separation hypothesis for DNA replication are described (experiments with heavy DNA, radioactive chromosomes, and the replication of DNA in vitro). This working hypothesis is presented: The coded information in DNA is transferred to RNA, which serves as a template for polypeptide synthesis. PHOTO CREDITS Dr. Asimov’s photograph by David R. Phillips, courtesy _Chemical and Engineering News_ Page 4 James German, M.D. 6 Bausch & Lomb, Inc. 12 James German, M.D. 20 Indiana University 24 Robert C. Filz, Air Force Cambridge Research Laboratories 25 J. K. Boggild, Niels Bohr Institute, Copenhagen University 26 Brookhaven National University 28, 31 Herman Yagoda, Air Force Cambridge Research Laboratories 41 Oak Ridge National Laboratory Footnotes [1]For more detail about cell division, see _Radioisotopes and Life Processes_, another booklet in this series. [2]This is more commonly known as “Mongolism” or “Mongolian idiocy” though it has nothing to do with the Mongolian people.