REGULAR MENDELIAN INHERITANCE 19-4 



larly and no fermenters appeared among the progeny. These data 

 proved that the ability to ferment galactose is controlled by a con- 

 ventional Mendelian gene. 



MENDELIAN INHERITANCE OF GENES AFFECTING FERMENTA- 

 TIVE AND VITAMIN-SYNTHESIZING ABILITY 



Burkholder's medium (Chapter 21) is an excellent diagnostic 

 medium for distinguishing pantothenate "synthesizers" from "non- 

 synthesizers," because a so-called nonsynthesizer grows rapidly 

 in this medium containing pantothenate, but requires weeks or 

 months to produce growth in its absence. However, genes affect- 

 ing vitamin synthesis are apparently transmitted in some pedi- 

 grees in a non-Mendelian way. The first pedigree on the inheri- 

 tance of "Vitamin-synthesizing" ability in Saccharomyces (Linde- 

 gren, 1945) failed not only to reveal regular Mendelian inheritance 

 of this ability but also of genes controlling the fermentation of 

 carbohydrates. In our selected inbred strains, the ability to fer- 

 ment galactose and maltose is transmitted in a regular Mendelian 

 manner, and the genes affecting the synthesis of paraminobenzoic 

 acid, pantothenate, pyridoxine, and thiamin are transmitted with 

 corresponding regularity. These genes are described as "affect- 

 ing" rather than "controlling" the synthesis of vitamins, because 

 we have not discovered any absolute deficiencies in yeasts. A so- 

 called nonsynthesizer of pantothenate eventually will grow in a 

 medium without the addition of pantothenate, although some cul- 

 tures do not begin growth until they have stood in the tubes for 

 nearly a month. 



In genetical analysis, it is relatively unimportant whether ab- 

 solute or partial deficiencies are dealt with; all that is required 

 is a clear-cut differentiation of the haploid offspring of a hybrid 

 into two different categories. This is easily effected in our pre- 

 sent yeast pedigrees by using Burkholder's medium with and with- 

 out added pantothenate. Genes affecting pantothenate and pyridox- 

 ine synthesis are easily diagnosed; the "nonsynthesizers" do not 

 begin to grow until a week after planting while the "synthesizers" 

 attain nearly full growth after 48 hours. The former may produce 

 a turbidity reading of between 200 and 300, while the latter still 

 show a reading of between and 5. After the tubes have been held 

 for two months it is difficult to distinguish them, but at 4 or 5 days 

 the difference is pronounced. Cultures differing in genes which 

 affect the synthesis of thiamin and paraminobenzoic acid show dis- 

 tinct differences at the end of 48 hours, but by the fourth day it is 

 difficult to tell them apart. However, any clear-cut segregation 

 of the progeny into two classes supplies the geneticist with an 

 adequate gene "marker." 



