DIRECTION AND FREQUENCY OF MUTATION 



229 



cases with more than a single individual. Table 6 gives the 

 number of mutant females and males appearing in each of these 

 five cases. In the selection lines, as shown in figure 2, each of 

 the eight mutations appeared as a single individual. There is 

 thus a great preponderance of single mutants. 



The facts in the case make it necessary to conclude that at 

 least five of the mutants represent changes occurring before the 

 end of the gonial divisions, probably early in oogenesis or sper- 

 matogenesis. On the other hand, it is not possible to draw any 

 definite conclusion regarding the relative frequency of the late 

 appearance of germinal changes because a considerable number 

 of the single mutants are probably merely chance survivors 

 among a group of similar sex-cells. However, because of the 



TABLE 6 



Bar stocks in which more than one mutant individual to full appeared simultaneously 



NAME OF STOCK 



B P C P S Bar 1132.4 



Vestigial sepia bar 1147.1 



Unselected bar 31 



Low white selection ¥7 168.1 

 Low white selection F7 168.1 



NUMBER OP 

 HETERO- 

 ZYGOUS 

 FEMALES 



Several 

 3 

 1 

 2 

 6 



NUMBER OF 

 FULL 



FEMALES 



NUMBER OF 

 FULL MALES 



Several 

 3 

 1 

 



large number of such single mutants, it is probable that at least 

 some of them represent mutations late in oogonial or spermato- 

 gonial history or during the growth or maturation periods. The 

 conclusion may therefore be reached with a considerable degree 

 of certainty that the mutations in the bar series may occur either 

 early or late in gonial history. 



It remains to consider the probability that both the affected 

 spermatozoa resulting from a single mutation occurring during 

 maturation may have given rise to mutant individuals. Such 

 spermatozoa would necessarily have to give rise to heterozygous 

 females, since they calTy the sex chromosome and since the chance 

 of meeting a mutant ovum is only one in 1500 squared. One 

 of the five cases with more than a single mutant individual con- 

 sists of two heterozygous females and may be due to the survival 



