446 E. J. WELLHAUSEN 



bining ability, one would expect to end up with about as many advanced 

 generation inbred lines which exceed the Si in combining ability with a 

 specific tester as lines which were below that of the Si. In other words, the 

 distribution in relation to the Si yield would follow that of a normal curve. 

 In Table 27.14 are given the distributions of the yields of Si and S3 top- 



TABLE 27.14 



FREQUENCY DISTRIBUTIONS OF Si AND S3 LINE-TOPCROSS YIELDS OF 12 



FAMILIES. CLASS IN WHICH Si LINE-TOPCROSS OF EACH FAMILY 



F.ALLS IS INDICATED BY NUMBER IN BOLD FACE TYPE 



Class Mid- 

 points (Yield 

 of Topcrosses 

 in % of 

 Checks) 65 75 85 95 105 115 125 135 145 155 165 175 185 195 205 



Family and 



Frequency 



Hgo. 9-4 12 7 



Ch. II 148 12 1 3 



M. 37-5 12 113 1 



Hgo. 4-5 11 4 2 3 3 1 



Hgo. 1-5 1 111 



Hgo. 3-4 1 2 



Hgo. 2-3 12 



Ch. II 187 12 



Hgo. 1-8 111 



Hgo. 6-11 1113 1 



Hgo. 3-5 12 2 11 



Ch. IV 146 2 1 



crosses of 12 families from the same race of corn (Chalqueno). All of the Si 

 and S3 lines within each family were topcrossed on an Si line from the race 

 Celaya, and all topcrosses were tested under the same conditions. The class 

 in which the Si topcross of each family fell is indicated by the number in bold 

 face type. It is evident from the table that the number of S3 lines from each 

 family tested are insufficient to show a normal distribution. However, in 

 nearly every case where more than three S3 lines were available for com- 

 parison, some were no better, some were significantly better, and still others 

 were significantly worse than the respective Si line in combining ability. It 

 appears, therefore, that in certain cases considerable increase in the yield of 

 specific combinations involving Si lines can be obtained through further in- 

 breeding and selection for specific combining ability. 



Further evidence that better yields can be obtained through the substitu- 

 tion of S2 or S3 lines in a specific Si combination is presented in Table 27.15. 



This table is divided into two parts. In column A is given the yield of each 

 specific single cross made with Si lines. In column B is given the yield of each 

 corresponding single cross of two advanced lines selected from the Si lines 

 given in column A in the second or third generation of inbreeding. Selection 



