Chapter 28 



THE BEARING OF THE DATA FROM YEAST GENETICS 



ON THE CURRENT CONCEPT OF THE GENE 



Tetrad analyses of hybrid yeasts nave yielded genetical data 

 which are not easily reconciled with current concepts of the gene. 

 These apparent contradictions of the generally accepted theoretical 

 structure have been given critical consideration in attempts to ex- 

 plain them on a conventional basis. Many of the obvious possibili- 

 ties have been explored and it appears that cytoplasmic inheritance, 

 rapidly mutating genes, modifying genes closely linked to the dom- 

 inant allele, somatic segregation in the diplophase, changes in ploidy, 

 and several other mechanisms could conceivably be involved. How- 

 ever, each of these explanations becomes extraordinarily complicated 

 or fails to deal adequately with some of the exceptional asci encoun- 

 tered in the pedigrees. The explanation offered below has been chosen 

 because it fits all the cases without any fundamental difficulty except 

 that it requires alteration of our apparently well-established views 

 concerning the stability of the gene. The fact that it is currently pre- 

 ferred does not rule out recourse to a more conventional view at a 

 future time if the data warrant it. 



Two new facts have been established by our work in yeast genetics 

 that are difficult to reconcile with current theory. The first is that 

 depletion mutations which occur, apparently spontaneously, dur- 

 ing vegetative reproduction and persist as stable variants on continued 

 vegetative reproduction, revert to the original type following sexual 

 reproduction. This phenomenon has been explained by assuming that 

 changes in gene-product to gene equilibria (Chapter 15, and fig. 28-1) 

 may produce stable vegetative variations. This hypothesis states that 

 different stabilized equilibria can exist between gene and gene-product 

 and that a different character may result from each equilibrium. 

 When an environmental effect changes the equilibrium and a new equil- 

 ibrium is established a new character appears. On this h3npothesis, 

 the variation from pink to white occurs when so much gene -product 

 X is lost that a crJlcal threshhold is passed and gene X can no longer 

 maintain the original concentration of gene -product in the cell. That 

 many characters other than color are affected is shown by the fact 

 that two white variants cannot be inbred. They are sterile when inbred 

 but fertile when outcrossed. 



Winge and Laustsen's phenomenon of degeneration through direct 

 diploidization is easily explained by this hypothesis: When the nucleus 

 divides without a concurrent division of the cytoplasm the equilibrium 

 of gene -product to gene is diminished by one -half since two genes are 



28-1 



