126 A MANUAL OF MENDELISM 



one factor in raising or depressing the yield. No doubt 

 it would be very difficult to do so, and it may be doubted 

 whether any serious effort has been made in this direc- 

 tion. Professor Nilsson-Ehle's object is to produce 

 high yielding varieties for Swedish farmers and, if this 

 can be done, as it can be done, without giving heed 

 to the Mendelian relationships of the factors or their 

 definite niunerical values, there is no reason why the 

 work in its economic aspect should be delayed out of 

 consideration for another aspect which is of much less 

 importance. But with the three pairs of factors already 

 mentioned and another which can be added we can 

 obtain some idea of what is occurring. The other 

 pair is tillering, or the production of side shoots, and 

 non-tillering. Experimenters with grain crops know 

 that tillering increases the yield, although it has not 

 yet been shown which is the dominant character. No 

 doubt there are other factors, some of greater some of 

 less importance. For instance, there are degrees in 

 resistance to winter and rust which must be dependent 

 upon the factors which produce them, but these may 

 be left out of account in the meantime. Nor need the 

 non-identification of the dominants and recessives 

 detain us, for the problem can be studied without this 

 having been done. 



In the case of the four pairs of factors now before 

 us, let us assume that those which raise the yields are 

 dominants and those which depress them recessives. 

 Then, if we symbolize them : resistance to rust = R, 

 non-resistance = r ; resistance to winter = W, non- 

 resistance = w ; resistance to lodging = L, non-resist- 

 ance = 1 ; and tillering = T, non-tillering = t ; the 

 following sixteen combinations are possible : 



