Gregory: efficacy of mutation breeding 



475 



larger variances were often associated with the smaller means. 

 This inverse relation is brought out more sharply by the distri- 

 bution of the standard deviations on the array of the F x gener- 

 ation means. The individual regressions which were found to be 

 negative and highly significant for the P1X + P2X and significant 

 for FoX are shown in Figure 2. The regression was much more 

 marked in PX than in FX, but the difference in slope was nonsig- 

 nificant at the 0.05 level. 



G 



PERCENT 



20- 



10- 



F| f am. X (F, gen) 



PI,P2 



i °7 



8'0 90 100 110 /0 



Figure 2. — Regression of genotypic C.V. (s G/ ~ X 100) of F s progenies 

 on Fj family mean in percentage of the mean of all treatments, x. 



In the light of the negative regression of s G on Fi generation 

 family mean in the irradiated material, the question arises as to 

 the relative magnitudes of the variances remaining in selected 

 high and low F 2 generation families. Will negative regression of 

 variance on mean have disappeared in the distribution of s G for F 3 

 generation progenies on F 2 generation family mean? The tests to 

 evaluate the means and variances of the high selections of F 2 and 

 F 2 X 2 as F 7 progenies in F 9 generation will not be conducted until 

 1961. However, four crosses from the experiment lost to the fall 

 storms of 1956 and involving some of the same parental material 

 as the crosses shown above have furnished material for making 

 such tests. 



