DEVELOPMENT OF THE HETEROSIS CONCEPT 



55 



yield was directly related to characters of the inbreds. These relationships 

 between the parents and their Fi crosses were somewhat larger than those 

 obtained by others with maize. Nevertheless, relationships were much 

 smaller than has been obtained in similar studies with self-pollinated plants. 



Richey (1945b) compared the yield of inbred parents in the S3 and S4 gener- 

 ations of selling with the mean yield of their single crosses from data taken 

 by Jenkins and Brunson. Similar comparisons were made between the yield 

 in top crosses and the mean yield in single crosses (see Table 3.6). 



Although for various reasons the r values are not strictly comparable, the 

 yield of inbreds was as strongly correlated with the mean yield of their 

 single crosses as the yield in top crosses was correlated with the mean yield 

 of single crosses. 



TABLE 3.6 



CORRELATION COEFFICIENTS FOR YIELDS OF 



INBRED PARENTS OR TOP CROSSES WITH 



MEAN YIELDS OF SINGLE CROSSES* 



* After Richey, after Jenkins and Brunson. 

 t S3 = three years selfed, etc. 



Comparison of Methods with Self- and Cross-pollinated Plants 



In self-pollinated plants it seems probable that the first natural step in 

 the utilization of heterosis normally may consist of the selection of available 

 parental varieties that in themselves produce the best combination of char- 

 acters. It seems important to continue breeding for the best combination of 

 genes that can be obtained in relatively homozygous varieties. Where hybrid 

 seed can be produced cheaply enough, or new methods can be found to 

 make crosses more easily, heterosis can be used to obtain from the hybrid an 

 advance in productivity over the homozygous condition. 



In cross-pollinated plants two general methods of breeding for heterosis 

 are now being widely utilized. One consists, as in maize, of the selection with- 

 in and between selfed lines and the use of single, three-way, or double crosses 

 for the commercial crop. The second general method consists of selecting 

 or breeding desirable clones of perennial crops. These are evaluated for com- 

 bining ability by polycross, or other similar methods, and the desirable clones 

 used to produce Fi crosses, double crosses, or synthetic varieties. 



