246 The Induction of Gene Mutations 



Drosophila melanogaster an average of two lethal mutations 

 occurs in every thousand chromosomes per generation, and a 

 dosage of 4800 Roentgen units will increase this rate about sev- 

 enty times. 



Although it would be far beyond the scope of this book to 

 catalogue all the visible mutations that have been induced by 

 X-rays or radium, a few examples will give the student some 

 conception of the kinds of phenotypes that result from irradi- 

 ation. In Drosophila melanogaster, mutations have been pro- 

 duced at a number of loci. For example. X-rays have caused 

 the gene w^ to mutate to some of the other alleles at that locus, 

 such as w (white), w^ (eosin), and w^ (apricot). Similarly, w'^^ 

 (coral), w^ (buff), w'^ (cherry), w^ (apricot), w^ (eosin), and w* 

 (tinge) have mutated to white and occasionally to some of the 

 other members of the series. All such mutations cannot be in- 

 terpreted as losses in the sense that the mutation is always to a 

 lower member in the series, for white has been known to mutate 

 to some of the higher members such as eosin, and eosin has 

 mutated to the dominant of the series, the wild type. It should 

 be noted that these mutations are similar to spontaneous ones at 

 the same locus. 



Whiting and his students have carried out extensive X-ray 

 experiments on the parasitic wasp, Habrobracon, and have listed 

 twenty-two mutant types which have resulted from irradiation 

 and are visible as phenotypic characters. These mutations in- 

 clude such abnormalities as pale, opaque, greenish-yellow body 

 and head color, flattened and curved hind feet, wings which fail 

 to fold over the body, wrinkled wings, black body color, dark 

 red compound eyes, drooping antennae, and small eyes (Fig. 

 75). The procedure in these experiments was to X-ray mated 

 females and test the offspring by breeding experiments. Since 

 the males develop from unfertilized eggs and therefore have 

 only one set of chromosomes, any mutations that should occur 

 in the unfertilized eggs would be detected immediately in the 

 male offspring. If a mutation was produced in either the egg 

 or sperm nucleus of the fertilized egg, the resulting daughter 

 would be heterozygous for the mutant gene. If the mutation was 

 dominant, it would immediately be detectable, but if recessive 

 it could be discovered only by subsequent breeding tests. 



