Questions and Problems 241 



tremendously increased rate when plants and animals are sub- 

 jected to radium, X-rays, ultraviolet light, and high temperatures 

 has been demonstrated on many occasions. This subject is dis- 

 cussed more fully in the next chapter. 



Effect of Other Genes on Mutability 



Rhoades has reported an interesting case of a stable gene 

 that becomes unstable in the presence of a nonallelic gene. In 

 maize, plants which have the genes B and PI are purple if A is 

 present but are brown if homozygous for a. If the gene Dt is 

 present in aa plants, the a gene undergoes frequent mutations to 

 A in somatic cells throughout the plant, producing plants having 

 husks and culms which are brown with longitudinal purple 

 stripes. The stripes are small, indicating that the mutations 

 occur late in somatic development. The gene a is an unstable 

 gene in the presence of Dt but not in homozygous dt dt plants, 

 for such plants never have the purple stripes. This frequent 

 mutation occurs not only in vegetative parts of the plant but 

 also in the aleurone layer of the fruit. Since this layer is part 

 of the endosperm tissue, each gene is present in three doses in- 

 stead of two. At the a locus is another allele a^ producing a pale 

 color. Plants which are a^a^a and also have the gene Dt have 

 only a third as many dots in the aleurone as aaa plants which 

 have Dt. This shows that Dt affects only the a allele and that 

 a greater number of a genes increases somatic mutations. 



Demerec found a similar gene in Drosophila melanog aster. In 

 studying the rate of spontaneous mutation of flies collected from 

 fifteen different localities all over the world, he found that the 

 frequency of spontaneous lethal mutations in the X chromosome 

 was much higher in a strain from Florida than in the others. A 

 genetic analysis showed that this higher rate resulted from the 

 presence of a recessive gene in the second chromosome. This 

 gene produces not only a high frequency of lethal mutations but 

 also an increase in the rate of visible mutations in a number 

 of other genes. 



QUESTIONS AND PROBLEMS 



1. In a plant, would a recessive gene mutation be detected more 

 easily if it affected the gametophyte or if it affected the sporophyte ? Is 

 the same true of a dominant mutation? 



