HYBRID VIGOR IN DROSOPHILA 489 



ways. The line crosses were 17, 30, and 62 per cent better than their inbred 

 parents. They were also 2 to 4 per cent better than the inbred X synthetic 

 crosses. Figure 29.3 shows these trends for the three types of progenies. 



Coeflicients were calculated for the like cross performances at different 

 generations. The synthetic X inbred lines had correlations for the 1, 8, 9, 16, 

 23, and 30th generations. Like numbers of comparisons were available in the 

 reciprocal crosses, inbred lines X synthetic. The correlations were similar for 

 generations and their reciprocals. The average for the thirty comparisons was 

 — 0.01. In terms of the data, the synthetic X inbred line cross of one genera- 

 tion gave no information on the relative performance of the same cross in a 

 succeeding test. The hybrids showed random variation within themselves, 

 but at the same time averaged out to be distinctly better than the inbreds. 



The inbred lines of the 21, 26, and 34th generations were crossed in all 

 possible ways. Again the correlations between the productivities of the like 

 crosses in different generations showed variation. The average correlation 

 was 0.25. The performance of the cross uniting two of these inbred lines did 

 have some predictive value for the performance of like crosses made subse- 

 quently. Again these hybrids showed most of the variation within the crosses 

 to be random, but that the yield level of the hybrids was significantly better 

 than the inbred parent lines. The over-all value for larger yield came as a 

 consequence of the cross rather than as an effect of specific cross differences. 



This fact is brought out in another way. The inbred crosses were analyzed 

 for general and specific combining ability, as described earlier, for the three 

 different generations of inbreeding, 15, 24, and 34. The average results were: 



General combining ability 12% 



Specific combining ability 5 



Residual variation 83 



These results are comparable with those presented earlier. General and 

 specific combining ability can be estimated for each line in the particular 

 crosses and experiments. The values can be compared as between the differ- 

 ent generations, to determine how consistent in combining ability is the be- 

 havior of each line. The correlations for these comparisons were as follows: 



15 and 24 15 and 34 24 and 34 



General combining abilities —0.02 —0.27 —0.17 



Specific combining abilities 0.13 0.02 0.26 



These correlations are so small as to indicate that combining abilities are 

 not consistent from one generation (in this case also season) to the next. The 

 hybrids are uniformly better than the inbreds in yield, but again the geno- 

 typic system does not appear to have a fixed reaction. The explanation of this 

 fact appears in Table 29.4 where a high experiment genotype interaction was 

 observed. It means that each genotype may react differently to different en- 

 vironment. As these environments change from place to place, season to sea- 



