248 The Induction of Gene Mutations 



the lemmas, and each has a long awn of the same length as 

 those on the lemmas. This mutation is of interest from the 

 morphological relationships and also because no cultivated 

 species or variety of barley known in either Europe or America 

 has outer glumes similar in appearance to the lemmas. 



Detection of Visible Mutations 



Several different methods have been used in applying X-rays 

 or radium for the production of mutations. In plants, dormant 

 seeds, sprouting seeds, flower buds, and pollen have been treated ; 

 experimenters on animals have irradiated sperm, unfertilized 

 eggs, and fertilized eggs. Somatic tissues have sometimes been 

 treated, but they, of course, would not produce inherited muta- 

 tions in animals. 



If a visible, nonlethal mutation is produced, it will usually be 

 detected more readity if it is a mutation to a dominant form 

 than if it is a recessive mutation. If seeds are irradiated, and a 

 certain recessive gene mutates to its dominant allele, this muta- 

 tion will be detected at once, provided that it is a gene that 

 affects the sporophyte. On the other hand, a similar mutation 

 of a dominant gene to the recessive allele will not be detected 

 until the plant is self-fertilized and the next generation is raised; 

 and if the plant is dioecious, or self-sterile, the recessive muta- 

 tion may not be detected for several generations. In animals 

 the problem is also a difficult one, since most of the animals 

 that have been studied cannot be self-fertilized, and, as most of 

 the mutations that have been produced are recessive, the detec- 

 tion of mutations may often be difficult. 



Many mutant types are strikingly different from and are easy 

 to distinguish from the nonmutant organism, but perhaps many 

 differ only slightly. Although these ^'slight" mutations can some- 

 times be detected, especially in lethals, it is the opinion of some 

 geneticists that a large number are never observed and recorded. 



In one peculiar strain of Drosophila melanogaster , known as 

 the "attached-X" strain, a very favorable situation is present 

 for the detection of recessive visible mutations which may be 

 produced in the X chromosome. In the attached-X stock, two 

 X chromosomes are attached to one another at the ends nearest 

 the centromeres. Because of this attachment, the two chromo- 

 somes must always go to the same pole during meiosis. At- 



