DEVELOPMENT OF THE HETEROSIS CONCEPT 



63 



tions, and also when heterozygous in near isogenic, homozygous, and highly 

 heterozygous backgrounds. 



The factor pair for Didi was studied in the near isogenic background of 

 inbred A 188, that of D^dx in the near isogenic background of A95-344, and 

 both factor pairs were studied in crosses between A 188 X A95. Particular 

 attention was given to total dry matter produced at various periods of growth 

 under field conditions and to the growth in length of the coleoptile and meso- 

 cotyl under controlled laboratory conditions. 



One comparison of the growth of the mesocotyl during a 12-day period 

 for DiDi and Didi on three different near isogenic backgrounds will be con- 

 sidered: the near isogenic background, A188, and the highly heterozygous 

 backgrounds of A 188 X A95 in the presence of D^D^ and D^d^, respec- 

 tively. While Di conditioned greater growth of mesocotyl in length than d\, 

 Dx conditioned less development of the mesocotyl in length than dx. 



The mesocotyl length of six strains consisting of comparisons of DiDi 

 with Didi on three dififerent backgrounds was taken as 100. The comparisons 

 are summarized in Table 3.16 and in Figure 3.1. 



It is apparent that the superiority of DiDi over Didi in mesocotyl length 

 becomes less in the highly heterozygous background than in the homozygous 

 background of A188. This may be more evident from the diagram in Fig- 

 ure 3.1. 



TABLE 3.16 



COMPARATIVE LENGTH OF MESOCOT- 

 \T. FOR SIX STRAINS OF CORN 



It seems of some interest that the differences between DiDi and Didi were 

 smaller in the highly heterozygous background than in the homozygous 

 background, and that in the presence of D^x that the differences were 

 further reduced over those in the presence of DxDx. It may be well to recall 

 that dx conditioned greater length of mesocotyl than Dx- 



Reference may be made to an explanation by Torssell (1948) of tlie decline 

 in green weight or length of stem in alfalfa in different generations of in- 

 breeding. It was not greatest in the first inbred generation. He suggests there 

 was a surplus of vigor genes in a heterozygous condition in the early genera- 

 tions of selfing, and that great loss of vigor was not observed until about I3 



