Discussion of Session IV 



I. J. JOHNSON 



Caladino Farm Seeds, Inc., Wheaton, Illinois 



Progress in plant improvement may be dependent upon the 

 incorporation into a new genotype of either one or many gene 

 pairs. Several examples can be given in which but a single gene with 

 major effects has meant the difference between success or failure in 

 the economic culture of a crop plant. The best examples of the 

 importance of single (or few) gene effects are the addition of disease 

 resistance lacking in otherwise agronomically desirable genotypes in 

 wheat, oats, barley, flax, and a number of other crops. Often, also, a 

 simple gene mechanism may control plant height, as in sorghum, and 

 a relatively few genes may control photoperiodic response or time of 

 maturity and thereby adaptation to a new environment. All favorable 

 simple gene affects need not be due to the action of dominant genes. 

 Many cases can be cited where the recessive is the "desirable" type 

 and the dominant the "undesirable" type. Hence, in the total realm 

 of plant improvement, one must not confine his thinking only to 

 quantitative characters for which expression usually is conditioned 

 by many genes, each with small effects and with action varying from 

 additive to epistatic. 



Throughout the long evolutionary processes in the development 

 of plants there has been accumulated a large number of mutant types. 

 These have been partially catalogued in several crops and maintained 

 as world collections. These mutant types provide a "gene pool" and 

 are the basic working stocks in many of our economic food plants. 



The mere existence of a known source of supply of genes to meet 

 specific needs does not imply that all of the genes useful in plant 

 breedino are now in existence. When a search for desired genes fails 

 to uncover sources of new characters desired in plant improvement, 

 speeding the process of natural mutation through artificial means 

 surely becomes an important part of plant breeding. During the 

 progress of this symposium, several examples (dwarf cotton, early 

 maturing Hybiscus, and rust-resistant Merion bluegrass) of new 



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