GENETICS AND CYTOLOGY OF SACCHAROMYCES 259 



Many yeast hybrids have been produced by mating sex cells carrying 

 chromosomes marked with biochemical mutant genes. The tetrads from these 

 hybrids have been analyzed by growing clones from each of the four spores 

 of a single ascus and classifying each of the spore-cultures. These exj)eriments 

 are direcl tests of the Memieliau theory. They have shown that excei)tions to the 

 Mendelian theory occur more frequently than was hitherto supposed. 



CURRENT STATUS OF IRREGULAR MENDELIAN SEGREGATION 



Tetrad analysis of triploid and tetraploid yeasts has revealed that some of 

 the irregular (not 2 : 2) segregations in hybrid asci arise from the fact that one 

 or both of the parents is diploid (Lindegren and Lindegren, 1951; Roman, 

 Hawthorne, and Douglas, 1951). Roman, Hawthorne, and Douglas have 

 concluded that all irregular segregations in Saccharomyces arise from the 

 segregation of triploid or tetraploid zygotes. We have recently completed the 

 analysis of segregation in diploid hybrids heterozygous for both MA /ma and 

 MG/mg. This analysis revealed that in many asci in which segregation of 

 MA /ma was 2:2 {MA MA ma ma), segregation of MG/mg was 1:3 {MG mg 

 mg mg). This finding excludes the possibility that the hybrid was either 

 triploid or tetraploid since segregation of both genes would have been equally 

 affected. The phenomenon has been explained as conversion of the MG gene 

 to mg in the zygote. This conclusion is further supported by evidence indicat- 

 ing that both genes are in the same linkage group. 



One hypothesis of the nature of the gene developed during the study of 

 irregular segregation seems to have some merit. This is the proposal that the 

 gene is a complex of many more or less loosely connected molecules rather 

 than a single macromolecule. In this view, the gene is composed of a series of 

 identical sites around the periphery of a more or less cylindrical chromosome. 

 These sites may be extremely numerous since they are of molecular dimen- 

 sions around the periphery of a thread easily visible under the microscope. 

 At these sites identical agents responsible for the action of the gene are 

 located. 



GENE DIVISION 



The concept of the gene as a bracelet of catalysts arranged on the outside 

 of the chromosome simplifies the concept of gene reproduction. When one 

 conceived of genes as macromolecules arranged like beads on a string, it was 

 difficult to understand how all the genes on a chromosome could divide 

 simultaneously. If, however, there are thousands of loci and chromogenes at 

 the site of a single gene on the outside of an otherwise inert chromosome which 

 is composed principally of skeletal material, any longitudinal splitting of the 

 chromosome will partition two qualitatively equivalent parts which may or 

 may not be quantitatively equivalent. The restoration of balance by inter- 

 dependence of the autonomous organelles may make precise division unneces- 

 sary. 



