FIXING TRANSGRESSIVE VIGOR IN NICOTIANA RUSTICA 



167 



in the manner employed in corn breeding, namely Jenkins' method, in which 

 the average of the four Fi's not contributing to the double cross was used. 

 These comparisons are shown in Table 10.5 for the three plant characters 

 studied. The differences between observed means and j)redicted values in the 

 nine comparisons made were all within the limits required for odds of 19: 1. 

 It was concluded that the double cross means for plant height, number of 

 leaves, and leaf length in iV. rustica could be predicted with a high degree of 

 precision by Jenkins' method. The results indicated that there were no 



TABLE 10.5 

 COMPARISON BETWEEN PREDICTED AND OBSERVED VALUES FOR PLANT 

 HEIGHT, NUMBER OF LEAVES, AND LEAF LENGTH IN THREE DOU- 

 BLE CROSSES INVOLVING FOUR VARIETIES OF N. RUSTICA 



A , B, C, D represent the parent varieties as shown in Table 10.1. 



marked interactions between the genes or gene sets from the four varieties 

 when combined in a variety of associations. 



To illustrate this point, let us assume that each parent is homozygous for 

 a different allele at each of two independent loci so that A = XX YY, B = 

 X'X'Fip, C = X^XWW\ and D = X^XWW^. The Fi's represent six dif- 

 ferent combinations of these alleles. Each double cross contains all four alleles 

 of each locus in four particular combinations. For example, the i)opulation 

 {AXB)XiCX D) is 1/4A'X2 _^ \/^xX' + \/^X^X- + \/AX'XHoi the X 

 locus and 1/4FF2 + 1/477^ + 1/47^72 + \/4:Y^YHor the Y locus. Sixteen 

 different combinations of alleles at the two loci are possible in this double 

 cross. Accurate prediction of the double cross value on the basis of only four 

 of these combinations, namely: Fi's AXC, AXD, BXC, and B X D, 



