GENE MUTATIONS AND EVOLUTION 61 



viduals having but one dose of any particular type of gene 

 material, and others having three doses (Figure 10). An 

 individual carrying a pair of mutated genes, one in each of 

 its chromosomes, is subjected to radiation. By this, a small 



B 



Figure 10. Diagrams of a small portion of the chain of genes, 

 showing how it is possible to compare the effects of one, two or three 

 mutated genes. A. Normal condition, two mutated genes (at right). 

 B. One mutated gene removed. C. Three mutated genes present. 



portion of one of the chromosomes, carrying the mutated 

 gene, can be removed, leaving the organism with but one 

 dose. Or by radiation a small piece of the chromosome con- 

 taining the mutated gene is broken off, and later by proper 

 breeding introduced into an individual that already has the 

 usual two doses; so that now it contains three. Thus it 

 becomes possible to compare the effects of one, two, or three 

 doses of a certain mutated gene. 



An example will illustrate the nature of the relations dis- 

 covered in such comparisons. The eye of the fruit fly is 



