GENETIC VARIATIONS 



357 



effect of the mutation is more pronounced than when two 

 mutated genes are present. This turned out to be the rule 

 for most mutations. Provided no unmutated gene of this 

 pair is present, a single mutated gene produces a greater 



-• •- 



A 



B 



Figure 70. Diagram to illustrate the relation of the number of 

 mutated genes (white) to the nature of the characteristic produced. 

 A, Two mutated genes present, the usual condition. B, But one 

 mutated gene present. C, Three mutated genes present. See text. 



alteration of the normal character than do two. By the use 

 of radiation it becomes possible to obtain individuals having 

 three examples of a certain mutated gene. This is done as 

 follows: By radiation a small piece of the chromosome con- 

 taining the mutated gene is broken off, and later by appro- 

 priate breeding, this small piece is introduced into an indi- 

 vidual that already has the usual two mutated genes. Thus 

 we obtain an individual containing three of the mutated 

 genes (figure 70, C). It is found that such individuals differ 

 less from the normal than those with but two of the mutated 

 genes, and still less than those with but one of the mutated 



