HYBRID VIGOR IN DROSOPHILA 



485 



and good or bad according to the genes which they contain. These genes 

 could be additive in effect making the genomes of uniform effect with other 

 inbred lines. Specific combining ability represents unlike combining ability of 

 the genomes from one race with those of a succession of other races. This 

 variation in hybrids could be due to different allelic distributions as comple- 

 mentary or epistatic reactions of the different gene combinations with which 

 the given genome was combined. The relative effects of general vs. specific 

 combining ability for a particular group of crosses may be measured in data 

 containing all possible combinations between a series of different inbreds. 

 Table 29.4 presents the egg productions for the possible hybrids of five inbred 

 races. 



TABLE 29.4 



EGG PRODUCTIONS OF 5 INBRED RACES AND THEIR CROSSES 



AVERAGE YIELDS FOR 5, 6, 7, 8, AND 9TH DAYS 



OVER 4 EXPERIMENTS 



Table 29.4 shows 14 of the race hybrids have higher average yields than 

 their inbred parent races. The average hybrid produced 2818.8 eggs, the aver- 

 age inbred 2242.2, or the increase over the average inbred was 25.7 per cent. 

 These data serve to re-emphasize the fact pointed out earlier, that charac- 

 ters built up of components of lesser characters generally show more heterosis 

 than observed for each of the components taken separately. 



The individual race crosses differed in their ability to unite into favorable 

 hybrids. Race D is evidently high in its general combining ability. The other 

 four races show about equal combining ability. Race D has this high general 

 combining ability even though its own productivity is rather low — 2173.4 — 

 eggs as against 2595.2 for another of the races. 



For individual flies the range in egg production was from to 146 eggs. 

 The zero egg producing flies are an important class which give an insight into 

 female sterility. We have dissected over 300 such flies. These observations 

 have led to the opinion that this class is the result of a variety of causes and 



