GENE CONVERSION 



26-32 



Asci containing more than two G spores may result either from 

 mutation of g to G occurring early in the galactose broth, or directly 

 in the zygote. (It should be recalled that adequate marker genes 

 are present to insure that they are true hybrids.) These con- 

 siderations show that while the concept of conversion in the hetero- 

 zygote involves mutation effected by gene-to-gene transfer of gene 

 material, the conversion which produces G mutants in the g clones 

 involves a long delayed mutation whose original cause may nave 

 been a similar transfer of genie material also occurring in the 

 heterozygous condition. 



HYBRIDS OF g BY gg CLONES 



Table 26-17 shows the results of mating stable g by gg clones. 

 Only five types of asci are possible and the fact that all five types 

 appear indicate that segregations are not involved but that the 

 chance of mutation occurring in the fermentation tube probably 

 determines whether or not a culture is diagnosed as gg. The pos- 

 sibility that the g g g g and the gg gg gg gg asci could have 

 arisen by illegitimate matings was excluded by the extensive use 

 of other marker genes. 



Table 26-1? 



Frequencies of 5 tetrad types occurring among 22 proven 

 hybrid asci, each obtained by matings of g x g^ clones. 

 Only these fiue types are possible, and representatives 

 of each appear. 



