20-8 THE YEAST CELL 



diagnose (especially in combinations with others), is relatively 

 unaffected by environment, does not diminish vigor to an unusual 

 extent, and usually gives regular ratios. "Bad" genes have one or 

 more of the opposite characteristics. Classical Mendelian genetics 

 is based on the analysis of data involving hybrids heterozygous for 

 combinations of "good" genes. Genetical analysis of material in 

 which only "bad" genes can be found is difficult if not impossible, 

 along classical lines; geneticists often say that such organisms 

 are "poor" genetical material and therefore cannot be studied prop- 

 erly. At least part of the difficulties involved in transferring clas- 

 sical genetics to useful purposes has arisen from the fact that 

 what plant and animal breeders consider their "best" organisms 

 are \;^*iat a classical geneticist would call "poor" genetical material. 



GRADED IMPAIRMENT OF GENE FUNCTION 



Our experiments with yeast and Neurospora suggest that the 

 difference between "good" and "bad" genes may be in the degree 

 of impairment of gene function that has occurred. When haploid 

 Neurospora microconidia are treated individually on agar plates 

 with ultraviolet and X-rays and then transferred to culture tubes, 

 a large number fail to grow but the remainder fall into four cate- 

 gories: (1) Some are normal and remain so. (2) Many are vari- 

 ants which revert to normal on the first of second transfer, sug- 

 gesting an injury which persisted for some time after the environ- 

 ment had become normal; a typical Dauermodifikation. (3) Many 

 variants are stable on vegetative reproduction, but most of these 

 apparently stable haploid mutants when mated with a haploid wild 

 type produce only wild type progeny, suggesting that repair on the 

 injury occurred in the heterozygote. (When we first discovered 

 this phenomenon we concluded that the injury was to the cytoplasm: 

 at that time, the absence of any segregation character was considered 

 proof for the view that no change in a gene had been involved. Analy- 

 sis of yeast genetics, however, now suggests that an impairment of 

 gene function had occurred which was repaired in the heterozygote.) 

 (4) A few variants were good Mendelian genes --easy to diagnose and 

 regularly segregating. On the present theory it is supposed that 

 "good" genes are produced by an irreparable injury to the gene. 



