THEORETICAL BASIS OF RUST RESISTANCE TESTING 311 



POPE: If I may answer, I don't think there is such a thing as a 

 single gene in complete control of any character. For instance, in the 

 genetics text books you find a pertinent example having to do with white 

 eye in Drvsophila, reported as a mutant from red eye and as a single gene 

 effect. This is a beautiful 3:1 example, but the student usually misses a 

 later paragraph pointing out that there is a series of about 40 or 50 

 known genes that have to function normally before the white versus red 

 eye alternative is possible. A week ago in Pullman, Washington, we had 

 an International Barley Conference. One paper that stuck in my mind was 

 presented by the von Wettsteins, a husband and wife team from Denmark. 

 So far they have deliberately induced some 500 mutants for "waxy" in 

 barley, and they have located 300 of these on 6 of the 7 chromosomes-- 

 several of them in allelic series. Every one of these mutants is differ- 

 ent. Their objective is 1,000 mutants, just for waxy. Now wax is a little 

 more complex than some simple chemical, but surely it is not the most 

 complicated plant character. If you can induce 500 mutants for waxy and 

 find 300 of them, there are probably 500± variants in all similar charac- 

 ters. Quit thinking about single genes, recognizing them when they go 

 by, and avoid them like a trap. 



BINGHAM: I agree, but you recognize the nature of waxy, or white 

 eye to start with. 



POPE: The principles behind simple 3:1 ratios is sound; but remember 

 underneath such obvious relationships is a great big base of genetic 

 variation, and it's mush as far as any recognizable ratios are concerned. 



