Heterosis 385 



eral mechanisms operate in different species or even in the same 

 organism, each of them producing a greater vigor in hybrids 

 between inbred hnes than in the inbreds, themselves, or in the 

 original stock from which the inbred lines were produced. 



One of the widely advocated theories maintains that the vigor 

 of hybrids is the result of their state of heterozygosity. Con- 

 tinued inbreeding develops strains which are highly homozygous, 

 but the different inbred lines may be homozygous for different 

 genes. When they are crossed, the hybrid is much more hetero- 

 zygous than either line. Thus the cross AA bb cc DD ee X «« 

 BB CC dd EE would produce a hybrid Aa Bb Cc Dd Ee, which 

 was heterozygous for a number of genes. This theory, then, 

 suggests that there is something inherent in the heterozygous 

 condition of a number of genes that brings about a greater vigor 

 in an organism and that heterogeneity in the general proto- 

 plasmic constitution is a favorable condition that stimulates 

 physiological reactions in general. Another theory assumes that 

 increased size is the result of the interaction of a number of 

 dominant genes for size. Thus, in the above cross, if each domi- 

 nant gene added 5 cm to the height of a 30-cm plant, and if 

 the genes were duplicate, cumulative, and dominant, the two 

 parents would be respectively 40 cm and 45 cm tall, but the 

 hybrid between them would be 55 cm tall. This theory of the 

 interaction of favorable, dominant genes also has had consid- 

 erable support. It is far beyond the scope of this book to pre- 

 sent a critique of the various theories, but it might be well to 

 mention one or two of the important modifications of them. 



Jones has proposed a modification of the dominance theory 

 by assuming that there might be a number of dominant genes 

 but that various groups of them might be linked together in the 

 several chromosomes. Objections to the theory of the accumu- 

 lation of dominant size-producing genes were that in generations 

 subsequent to the F2 races would segregate out which are homo- 

 zygous for all the positive genes for growth or for their negative 

 alleles and that the distribution in the F2 would be unsym- 

 metrical for the characters which show^ed heterosis in the Fi. If 

 several size genes are on the same chromosome, and if there are 

 several such chromosomes, and if the positive size genes of each 

 chromosome are different from those of its homologue, the Fi 

 will show heterosis, the F2 will be symmetrically distributed, and 



