358 GENETICS 



genes. The individuals containing three mutated genes may 

 be almost completely normal. 



The general rule may be expressed as follows: A given 

 mutated gene may be present in either one, two, or three 

 "doses" (figure 70). The greater the number of doses the 

 less the effect of the mutation. 



Certain examples will illustrate these effects. Cherry is a 

 recessive mutation of the gene at 1.5 in the X-chromosome 

 of Drosophila. The effect of this mutation is to lighten the 

 color of the eye. When two doses of the mutation are 

 present, the color is lightened a certain amount. When only 

 one dose is present, the color is lightened still more. When 

 three doses are present, the color is hardly lightened at all. 

 This is the usual result with any of the mutational eye-colors. 



These relations show that what the mutation does is to 

 partly inactivate the gene, so that it no longer produces so 

 great an effect as before. In the case of eye-color, less color 

 is produced in the mutated gene. But two of the mutated 

 genes produce more color than one, and three produce still 

 more; almost as much as does the unmutated gene. 



This method of action has been demonstrated for many 

 different types of mutation. Thus, the mutation scute de- 

 creases the number of bristles on the body. If but one scute 

 gene is present, the number of bristles is considerably less 

 than normal. If two scute genes are present, the number of 

 bristles is greater. If three are present, the number of bris- 

 tles is almost completely normal. 



In some cases it is not at first obvious that the effect 

 of the mutation is a reduction in action as compared with 

 the normal gene, but experiments of the type just described 

 show that this is indeed the case. The body of Drosophila 

 carries a large number of simple bristles. By mutation of a 

 certain gene, some of the bristles are caused to be forked in- 

 stead of simple. But this forked condition is most marked in 

 individuals that have but one of the mutated genes (the 



