QUINBV: MUTATIONS IN SORGHUMS 205 



the "wild type" being early maturing grows out of the fact that the 

 alleles for late maturity act like dominant inhibitors. 



Singleton: The dwarf 3 (dw 3 ) appears like the effect of pseudo alleles 

 similar to the case of Star-asteroid eye characters in Drosophila analyzed 

 by E. B. Lewis who found crossing over within the locus. When two 

 wild-type alleles were located on a single chromosomal strand (cis posi- 

 tion), the phenotype was wild. If a similar situation exists for dwarf 3 

 in sorghum, a tall plant would be expected following a rare crossover 

 within the locus. If this is the proper interpretation, it might be cor- 

 rected by radiating the recovered tall plants, deleting the + allele so that 

 crossing over would not give rise to a tall (+) plant. 



Nuffer: I would like to suggest a method of solving the problem of 

 instability of the dw 3 locus. This situation seems especially well-suited 

 for the application of ionizing radiation. Since the reversion of dw 3 to 

 Dw 3 prevents the production of true breeding dw l dw 2 diu 3 dw 4 lines, a 

 likely procedure would be to induce a deficiency at the Dw 3 locus. This 

 could best be done by subjecting seed of a reverted dw x div. 2 Diu 3 dw 4 

 line to X-radiation. Plants from the treated seed would then be selfed 

 and a search made in the progenies for 4-dwarf plants. Since X-ray- 

 induced changes are usually irreversible, the new dw 3 should be stable. 



Quinby: Dr. W. R. Singleton has also suggested this same procedure 

 and I shall send him seed of several reverted dw-^ dw 2 Dw 3 dw± strains 

 to be X-rayed and returned to me. 



Mehlquist: Relative to your suggestion that the mutation for disease 

 resistance has occurred many times, is it not reasonable to assume that 

 this gene has existed for a long time but did not show until the nonre- 

 sistant plants were exposed to the disease? 



Quinby: Perhaps the genes for resistance might have been carried along 

 in the populations, but resistance was not always found in a variety 

 the first time it was grown in a diseased field. Also, resistance was found 

 in many different varieties, or the same variety at different locations, 

 and the incidence of resistant plants was quite low, sometimes as low 

 as two or three plants to 5 or 10 acres. 



