[THOMPSON] RIPENING PERIODS IN WHEAT 79 



From the Fi results alone it was natural to conclude that we were 

 dealing with a simple case of dominance of lateness over earliness. 

 But when the F2 results were obtained (see tables) it became clear 

 that this was not the case. These results show that we are really 

 dealing with a case of blending inheritance, most of the F2 plants 

 being intermediate between the parents. If this is true the Fi plants 

 should also be intermediate according to all experience with blending. 

 The difficulty thus presented will be discussed in later pages. 



Second Hybrid Generation. — In the second hybrid generation of 

 each cross the tables show that the majority of the plants are inter- 

 mediate between the parents and that there is considerable variation 

 in either direction. It will also be observed that the plants of this 

 generation have the frequency distribution of a regular curve of 

 probability. There is no indication in any of the crosses of a heaping- 

 up of the individuals in any position except the intermediate one. In 

 other words there is no sign of dominance or segregation such as is 

 found in simple cases. Whether the F2 results support the contention 

 that there is a real segregation involving many factors, will be dis- 

 cussed later. 



The range of variation among the Fo plants is in each case much 

 greater than that of either parent. Unfortunately no comparison 

 can be made with the Fi generation because of the small number of 

 plants available in the latter case. In general the F2 variation extends 

 practically from the condition of the earliest plants of the early 

 parent to that of the latest plants of the late parent. In some cases 

 it extends beyond the parental conditions at both extremes. In all 

 cases it extends at least as far in both directions as the mean parental 

 conditions. 



It will also be observed that the more the parents resemble each 

 other the more likely is the F2 variation to extend beyond the parental 

 extremes. In the crosses represented in the tables IV, IX, XII and 

 XIII, there is a difference of only a few days between the parental 

 conditions, and the F2 variation covers practically the whole range 

 of parental variation and even extends beyond it in IX, XII and 

 XIII. But in crosses involving parents more unlike the F2 variation 

 does not reach the parental extremes (tables V, VI, VII, VIII, X, XI). 

 This is true because the range of F2 variation does not increase in pro- 

 portion to the difference between the parents. As the parental differ- 

 ences increases the range of F2 variation also increases but not so much 

 as the parental difference. These facts are shown in table XIV. 



