3l8 AN INTRODUCTION TO MODERN GENETICS 



initial advantage conferred on it by the greater size of seed formed on 

 outcrossing, that is to say, by an increased growth rate of the hybrid 

 embryo between fertilization and seed formation. This, however, seems 

 unlikely to be a general explanation. Sinnott and Houghtaling^ have 

 shown that although the different sizes of squash fruits are often a 

 consequence of differences which can be found at a very early stage of 

 development, in other cases they are due to different amounts of grov^ 

 of primordia which were originally of the same size. And since there 

 undoubtedly are some hereditary differences of the growth rates in 

 later stages of plant development, it is difficult to see why they should 

 be denied any part in producing heterosis. Finally, the theory cannot 

 be applied to animals, in which there is no sudden check in develop- 

 ment comparable to seed formation. Castle and Gregory ^ have shown 

 that the eggs of rabbits of a large race cleave faster than those from a 

 small race from the very earliest stages, and tliis higher growth rate 

 continues throughout the Hfe of the animals. 



Heterosis has already found practical appUcation, particularly in 

 maize breeding. Inbred Unes are prepared, which although homozygous 

 for some deleterious factors and therefore weakly in constitution, are 

 nevertheless cleared of the most damaging factors which are completely 

 lethal in homozygous condition. On crossing two such lines, extremely 

 sturdy and productive plants are raised in Fi. A further increase in 

 yield can be obtained by crossing two hybrid Fi's, giving a "double- 

 cross" F2, but beyond that it does not seem profitable to go. 



Hybrid vigour can be perpetuated in plants which propagate vegeta- 

 tively, and valuable hybrid trees (e.g. poplars) have been prepared in 

 this way. 



In crosses between animal varieties, hybrid vigour is usually found, 

 and advantage is often taken of it in raising beef cattle, for instance. A 

 similar enhancement of vigour is sometimes found in wider crosses, 

 between different species, as in the well-known case of the mare-ass 

 cross, which produces mules, and in various crosses between bovine 

 species. But in such cases it is by no means clear that the hybrid vigour 

 can be explained entirely as a result of bringing into the genotype 

 dominant allelomorphs of deleterious factors, since it is probable that 

 the genotypes differ more profoundly than in the possession of different 

 allelomorphs at the same loci. One must suppose that in those cases in 

 which hybrid vigour is found in species crosses, it is a more or less 

 fortuitous result of a happy combination of the two gene complexes ; 



* Cf. Sinnott and Dunn 1935. ^ Castle and Gregory 1929. 



