The Nature of Mutations 

 in Terms of Gene and Chromosome Changes 



JOHN R. LAUGHNAN 1 



University of Illinois, Urbana, Illinois 



For the breeder, variability of the biological entity, regardless of 

 its source, is essential. But considered from an operational stand- 

 point, his success in the selection of improved strains has depended 

 to only an insignificant degree, or not at all, on an understanding of 

 the ultimate basis for that variability. The fact that geneticists have 

 so far been unable to provide a convincing demonstration of intra- 

 genic mutation means that, for the breeder, the question of whether, 

 in a particular instance, variability is attributable to intragenic or 

 extragenic causes must for the time being remain academic. Mainly 

 then, his approach to the problem has been to deal with the potential 

 variability available in his source materials and to develop operation- 

 al techniques aimed at manipulating this variability to the advantage 

 of his program. Thus, the main impact which genetics has so far had 

 on breeding programs has been to provide general techniques based 

 on the principles of transmission genetics and, in my opinion, it is 

 our lack of understanding of the physiological actions and inter- 

 actions of genes, including their capacities to change, which is mainly 

 responsible for differences of opinion and earnest debate over the 

 breeding method to invoke in a particular instance. 



Nor is it always clear that a certain apparent degree of success in 

 an improvement program is a function of the breeding technique 

 employed. For one thing, superiority is often relative, difficult to 

 assess, and sometimes embarrassingly short-lived. Then, too, nature 

 is on the side of the breeder, and while he is therefore reasonably 

 confident that he will finish with no worse than he started, he must, 

 by the same token, ofttimes concede that his efforts may have been 

 quite passive, alongside those of nature, in achieving a proved 

 measure of success. 



Currently Guggenheim Fellow on leave, Biology Division, California Institute of 

 Technology, Pasadena, California. Work reported here was aided by a grant from the 

 National Science Foundation. 



