166 



HAROLD H. SMITH 



The Fi's were taller and had larger leaves, on an average, than the mid- 

 parent. It was concluded, therefore, that a preponderance of dommant+ 

 genes was mvolved in determining differences in plant height and leaf length. 

 In the development of the parent varieties, selection resulted in the accumu- 

 lation of dominant-f modifiers, as is usually the case in naturally cross- 

 pollinated plants. 



The result with the character leaf number was different in that the Fi had 

 fewer leaves, on an average, than the mid-parent. Evidently, in the evolution 

 of the varietal gene sets, there had been accumulated a preponderance of 

 recessive+ modifiers (or dominant genes ior fewer leaves) at the loci by which 



TABLE 10.4 



DIFFERENCE BETWEEN THE Fi AND MID-PARENT (Fi - MP) AND THE 

 POTENCEt RATIO IN INTERVARIETAL HYBRIDS. BOTH V.ALUES ARE 

 EXPRESSED AS THE AVERAGE FOR EACH VARIETY IN CROSSES WITH 

 THE OTHER THREE VARIETIES* 



* 1947 data. _ _ _ 



t Potence = Fi-MP/kiPr-Pi). 



the parents differed. There can be little doubt that selection for mafty leaves 

 was practiced in producing the parent types. This is especially true for Olson 

 68 which was developed from hybrid origin by the late Mr. Otto Olson 

 (Smith and Bacon, 1941) by selection for plants yielding large amounts of 

 nicotine. In crosses with Olson 68, the Fi was consistently below the mid- 

 parent. This result, interpretable as due to an accumulation of a preponder- 

 ance of recessive genes for the character favored by selection, might be ex- 

 pected occasionally in naturally self-pollinated plants. Dominance is of less 

 importance here than in cross-pollinated organisms, since selection is largely 

 a matter of sorting out superior homozygous combinations. 



The 1949 results (Table 10.2) on Olson 68 X 34753 were consistent with 

 those of 1947 discussed above. 



Double Crosses 



The three possible double crosses involving all six Fi hybrids of four varie- 

 ties were grown in 1947 in order to obtain evidence on genie interactions by 

 comparing experimental results with predicted values. The latter were made 



