RHOADES: DISCUSSION OF SESSION I 51 



Rhoades: If chromosomes are multi-stranded it is, of course, difficult to 

 understand how a mutational event consisting, for example, of a base 

 substitution could occur simultaneously in all of the component strands 

 and thus be immediately expressed. However, the genetic evidence for 

 whole-body mutations occurring in gametes cannot be questioned. This 

 has been interpreted as indicating that the chromosome of higher organ- 

 isms consists of a single double helix of different DNA molecules which 

 are linearly arranged and possibly connected by protein links. If the 

 genetic data are deemed more reliable than the cytological studies, then 

 one favors the latter concept of chromosome structure. This conflict has 

 not been resolved. 



Caspar: With respect to the remark that corn does not respond in the 

 same way as other organisms in radiation experiments, I should like to 

 state that in some preliminary data of ours from an experiment with corn 

 similar in design to the ones the Russells have done with mice in which 

 the male gametes carrying dominant marker genes are radiated at different 

 stages of gametogenesis and crossed on multiple recessive females, our 

 results are quite similar. We find that the controls and material radiated 

 after meiosis consist of losses of adjacent marker genes and most all the 

 mutants also carry associated sterility effects, while the mutants from 

 material radiated prior to meiosis consist of single gene losses and the 

 mutants do not carry associated sterility effects. 



