62 GENETIC VARIATIONS 



normally a deep red in color. When a mutation has occurred 

 in a certain gene that is near the left end of the X-chromo- 

 some, this causes the eye to change to a lighter color, known 

 as eosin. The effect of the mutation is thus to lighten the 

 color of the eye. This lighter color, being recessive, is mani- 

 fested when two of the mutated genes are present together 

 (Figure io,A). 



Removal of one of these two mutated genes, leaving but 

 one dose of the mutated material (Figure lo, B) would 

 naturally be expected to decrease the effect of the mutation, 

 causing the color not to be lightened so much as when two are 

 present. But when this was done, it was found that the light- 

 ening effect was increased instead of diminished; with but 

 one mutated gene the eyes are lighter in color than with two. 

 Two mutated genes give a color nearer the normal red than 

 does one mutated gene. And when a third mutated gene was 

 added (Figure lo, C), the eye color became still darker, 

 approaching closely that of the normal red eye. 



It thus becomes clear that the effect of the mutation has 

 been to reduce the normal action of the gene, so that more 

 doses of the mutated gene are required to produce the deep 

 color than are required of the unmutated gene. The mutated 

 gene has the same kind of action as the normal gene, so far 

 as it goes, but its action is imperfect, incomplete. For it is 

 found that the presence of only one dose of the mutated gene 

 leaves the individual very different from the normal unmu- 

 tated individual. If two are present, the result approaches 

 more nearly that of the normal gene, while if three of the 

 mutated genes are present, the individual becomes almost 

 like one in which there is no mutation. 



It will be recalled that this eye gene is one that has been 

 mutated in many different ways, yielding many different 

 colors. One of the other mutations, giving a still lighter color. 



