DIRECTION AND FREQUENCY OF MUTATION 225 



any such periodicity. However, there have been no adequate 

 data bearing on a possible periodicity in appearance of a partic- 

 ular single mutation. This has been due in part to the infre- 

 quency of most mutations and in part to lack of concentrated 

 attention upon single mutants for considerable periods of time. 



In the present series of observations the number of mutations 

 observed at a single locus is considerable, but the period of time 

 is comparatively short. In the observations made upon the 

 stocks shown in figure 5 large numbers of individuals were ob- 

 served during a period of six months. The mutations are dis- 

 tributed fairly uniformly throughout this period. In the case 

 of the selection lines shown in figure 2 the observations extend 

 over twenty-two months, but include a much smaller number ot 

 individuals than the other series. An examination of both 

 series of data and a consideration of the fact that from the time 

 of its origin bar has been subiect, to 'contamination' by full leads 

 to the conclusion that no periodicity has been demonstrated. 



In this connection it ip well to consider that the environmental 

 conditions were kept a? uniform as possible, so that there is no 

 basis for a periodicity due to environmental periodicity. The 

 bearing of the results is upon the possibility that the germplasm 

 undergoes periodic changes in condition independently of the 

 environment. No such changes have been demonstrated. This 

 uniformity in the germplasm is in line with the results of com- 

 parisons between rate of mutation in stocks of different origin 

 and in those with different accessory factors, neither of which 

 seem to affect the direction or rate of mutation. 



10. Does mutation occur in both male and female parents? If 

 the internal change in a sex eell producing a mutant in this series 

 occurs in the male parent, a heterozygous daughter results. If 

 it occurs in the female parent, either a mutant son or a heterozy- 

 gous daughter appears. If the change may occur in either 

 parent, both heterozgyous daughters and mutant sons are to be 

 expected, but with a pt-eponderance of the heterozygous daughters 

 over the full sons. If it is postulated that an X chromosome has 

 the same liability to mutate whether located in a male or in a 

 female, the chance of mutation in any individual female sex-cell 



