Chapter 19 

 REGULAR MENDELIAN INHERITANCE 



A GENE -CONTROLLED ADAPTIVE ENZYME 



The fermentation of galactose by S. cerevisiae is an adaptive 

 reaction (fig. 17-1). Galactozymase is not present in nmeasurable 

 amounts in cells grown in glucose, and only appears in cells placed 

 in galactose after an incubation period of from 3 to 8 hours. Cells ' 

 which have been adapted to ferment galactose lose this ability when 

 removed from the substrate and have to be readapted to use it fer- 

 mentatively. The following genetical analysis shows that the dif- 

 ference between an adaptable culture and an unadaptable culture is 

 controlled by a single Mendelian gene. The ability to adapt is not 

 transferred through the cytoplasm as the adaptive process might 

 suggest; cells carrying the recessive gene are incapable of fer- 

 menting galactose. 



THE ORIGIN OF A NONFERMENTER OF GALACTOSE 



The demonstration that a given property is under genetic con- 

 trol requires the production of a hybrid between an individual pos- 

 sessing the property and one without it and the demonstration of 

 regular segregation of individuals with and without the property 

 among the progeny of the hybrid. However, the ability to ferment 

 galactose is widely distributed in the genus Saccharomyces, and 

 we had great difficulty in finding a suitable nonfermenting haplo- 

 phase capable of producing hybrids in which we could study the in- 

 heritance of this character. We obtained from Dr. L. J. Wickerham 

 a culture of S. microellipsoideus (NRRL, No. 210) which was a 

 relatively weak fermenter of both galactose and melibiose. Four 

 asci dissected from S. microellipsoideus produced 9 cultures which 

 fermented galactose slowly. A hybrid (i x z, Table 19-1), Linde- 

 gren and Lindegren, 1947) was made between one of these and S. 

 cerevisiae (F.L.D.), which ferments galactose vigorously. The hy- 

 brid was brought to sporulation and the single ascospore (No. lA) 

 grown from it was found to be incapable of fermenting galactose. 



TETRAD ANALYSIS OF AN aG/ag HYBRID 



The nonfermenter was mated with a haplophase galactose fer- 

 menter originating from S. cerevisiae (Mrak, No. 93). An a -mating 



19-1 



