GENETICAL CHARACTERS 11-8 



GALACTOSE -MELIBIOSE COMBINATIONS 



The fermentation of galactose and the hydrolysis of melibiose 

 are controlled by two independently segregating genes. This means 

 that four kinds of progeny are produced when a gamete capable of 

 fermenting both galactose and melibiose (G ME) is mated to one 

 incapable of fermenting either sugar (g me). 



PRODUCTION OF CO^ WHEN DIFFERENT SUGARS ARE 

 USED IN THE FERMENTATION TEST 



The fermentation test can also be made in Smith tubes (fig. 11-3) 

 by inoculating a nutrient broth containing a single specific carbo- 

 hydrate. After incubation the following results may occur: 



1. Rapid fermentation. Growth occurs within 24 to 48 hours 

 and the tubes fill with gas within that time. 



2. Slow fermentation. Growth occurs slowly and the tubes are 

 filled with gas within from 5 to 20 days. 



3. Growth occurs slowly but no gas appears in the tube. (This 

 may occur frequently if the concentration of sugar is less than 1 

 per cent.) 



4. No growth occurs and no gas appears. 



In the genetical analysis of the haploid members of our pedigrees 

 we have encountered phenomena 1 and 4 most frequently. The 

 other types occur rarely and the distinction between types 1 and 

 4 is controlled by a single pair of genes in many pedigrees. We 

 have called type 1 "fermenters" and type 4 "nonfermenters." Un- 

 til recently we thought that type 4 was incapable of producing the 

 specific enzyme. However, when the "nonfermenters" are placed 

 in Smith tubes on mercury they produce gas albeit slowly. This 

 means that the single gene difference between "fermenter" and 

 "nonfermenter" does not involve presence or absence of the ability 

 to produce the enzyme but that a gene pair exists whose dominant 

 allele controls a high rate of enzyme production while its opposite 

 number recessive allele is incapable of producing sufficient enzyme 

 to register as a positive test in the Durham tube (fig. 11-2). How- 

 ever, cells carrying the recessive gene are able to produce enough 

 enzyme to register a positive test in a Sn ith tube on mercury 

 (fig. 11-4). 



The "slow" fermenters (type 2 present still another problem. 

 Winge and Roberts have described "slow" Termenting cultures of 



