142 EDGAR ANDERSON AND WILLIAM L. BROWN 



lOD X IID we would expect relatively little heterosis within either of the 

 crosses. Accordingly, when we crossed 2F X IID we would rate this cross 

 as having high specific combining ability. On the other hand, had we origi- 

 nally crossed 2F X lOD and IID X IF, then there would probably have 

 been almost equally great heterosis in each of the crosses. Had these been 

 used as a basis for comparing the heterosis of 2F X IID, then our notion as 

 to the amount of heterosis in these crosses would have been very different 

 than it would have been had comparisons been made with IF X 2F or 

 lOD X IID. 



If the germ plasms of the two main races of maize involved in Corn Belt 

 dents are still partially intact as a result of linkages, it should be possible to 

 classify inbreds on the basis of morphological differences according to their 

 flint and dent tendencies. If this can be done, and if genetic diversity is im- 

 portant in bringing about a heterotic effect in hybrids, one should be able to 

 predict with some accuracy the relative degree of heterosis to be expected 

 from crossing any two inbred lines. With this hypothesis as a background, a 

 series of experiments was started three years ago to determine whether or not 

 hybrid vigor in maize, as expressed in terms of grain yield, could be predicted 

 on the basis of morphological differences of inbreds making up the Fi hybrids. 



Fifty-six relatively homozygous inbred lines consisting of eighteen 

 U.S.D.A. or experiment station lines, and thirty-eight strains developed by 

 the Pioneer Hi-Bred Corn Company were scored for the following character- 

 istics: row number, kernel length, denting, development of husk leaf blades, 

 number of secondary tassel branches, glume length, and chromosome knob 

 number. For each of these characteristics the two extremes in the eastern 

 United States are to be found in Southern Dents and Northeastern Flints. 

 At least twelve plants of each of the fifty-six inbreds were scored, and these 

 scores were then averaged to give a mean value for the line. The resulting 

 means were translated into numerical index values, in which a low value 

 represents Northern Flint-like tendencies, and a high value Southern Dent- 

 like tendencies. For example, the mean row number values for the inbreds 

 studied ranged from 11.2 to 19.5. These were arranged in the following index 

 classes. 



1 2 3 4 S 6 7 



11.2-11.7 11.8^12.3 12.4-12.9 13.0-13.5 13.6-14.1 14.2-14.7 14.8-15.3 



8 9 10 11 12 13 14 



15.4-15.9 16.0-16.5 16.6-17.1 17.2-17.7 17.8-18.3 18.4-18.9 19.0-19.5 



Index values for the other characteristics were arranged similarly, and 

 from the individual characteristic inbred indices (each being given equal 

 weight) a total "Inbred Index" was determined as is shown by example in 

 Table 8.2. 



After index values had been determined for the inbreds, single cross combi- 



