428 E. J. WELLHAUSEN 



All are races which have come into existence in more recent times and all 

 have rather complex pedigrees as shown in Figures 27.1-27.4. So far only the 

 outstanding varieties of each race have been inbred. It was concluded early 

 in the breeding program that inbred lines for immediate use in a particular 

 zone might best be obtained from the high yielding varieties adapted to that 

 zone. It is entirely possible that in the future better hybrids may be obtained 

 through the utilization of a wider base of germplasm. The latter procedure 

 would require more time and more adequate testing. It would involve the ex- 

 traction of the favorable yield factors from several races and their conver- 

 gence into a synthetic variety or hybrid along with the proper governing genes 

 for a given environment. 



In the inbreeding program it has become apparent that vigor in the origi- 

 nal plant or first generation selfed progeny (S]) can be used to a considerable 

 extent as a measure of the number of favorable vield genes with which the 

 particular plant or line has been endowed. But vigor becomes less and less 

 useful as an indicator of the number of yield genes in a particular line in the 

 second, third, or fourth generations of inbreeding because of a greater num- 

 ber of bottleneck genes that are fixed with successive generations of inbreed- 

 ing. In advanced inbred generations, two lines might be greatly different in 

 vigor, yet equal in combining ability with a specific tester, because of simi- 

 larities in their yield gene complex. As lines, they may be different in relative 

 vigor because of certain different specific bottleneck genes or loci that mask 

 the expression of genes or loci for growth and development. 



The Mexican corn program not only involves the actual improvement of 

 corn, but also the development and training of young corn breeders. In order 

 to demonstrate more vividly that considerable effort may be saved by select- 

 ing only the best and most vigorous plants or lines for further work, several 

 students visually classified the inbred lines available after one generation of 

 selfing from five different varieties into four classes on basis of vigor. The 

 most vigorous lines were classified as A lines. Those somewhat less vigorous 

 or desirable were classified as B, and so on, with the least vigorous lines classi- 

 fied as Z). 



Although classifications were originally made into four categories, the D 

 lines were discarded without further consideration. Of the remaining lines 

 saved from each of the five different varieties, 3-6 per cent in each were clas- 

 sified a.s A, 12 to 15 per cent as B, and the remaining 79 to 85 per cent as C. 

 All were topcrossed to a common tester, but only the topcrosses involving A 

 and B lines were finally included in yield tests because of lack of space. Com- 

 parative yields of the A and B topcrosses are summarized in Tables 27.1-27.3. 



Table 27.1 shows the results obtained with A and B lines from a variety 

 known as Leon CrioUo topcrossed on the variety Urquiza and tested at three 

 locations. At each place the average yield of the topcrosses involving the A 

 lines was slightly higher than those involving B lines. But what is more im- 



