GENETICAL CHARAaERS 11-6 



ment on the character. The' rate of growth is probably considera- 

 bly influenced by the ability to synthesize amino acids, vitamins, 

 and other essential substances. As will be pointed out subsequently, 

 it is very difficult to find yeasts completely unable to sjnithesize any 

 any one of these substances, suggesting that the different cultures 

 vary primarily in the quantitative rate of synthesis rather than in 

 absolute deficiencies. Quantitative differences of this type are gen- 

 erally those most important to the breeder attempting to achieve 

 improvements in animals or plants for industrial or agricultural 

 uses, but they are used only rarely by the classical geneticist be- 

 cause they are too difficult to analyze. 



LETHAL GENES 



Winge and Laustsen (1939) described a lethal gene in Saccharo- 

 mycodes ludwigii. Lethal genes cause the death of the haploid seg- 

 regant or the diploid homozygote. For this reason they cannot pass 

 through the haplophase in S. cerevisiae, although diplophase cul- 

 tures might accumulate a large number of lethal genes by mutation 

 and survive because the normal allele of the lethal would suffice to 

 maintain the organism. Lethal genes are very common in Droso- 

 phila and can be transmitted through either the egg or the sperm 

 because neither of these carries on an independent existence. As 

 soon as fusion occurs the normal allele of the lethal gene supplies 

 the essential substance, the deficiency of which causes the homo- 

 zygous lethal to die. Dodge and Lindegren showed that haploid nu- 

 clei containing lethal genes are easily maintained in heterokaryons, 

 and Beadle and Coonradt exploited this method in maintaining bal- 

 anced Neurospora stocks on minimal medium. A thallus containing 

 two kinds of nuclei one unable to synthesize thiamine and a second 

 unable to synthesize pantothenate could survive in a medium lacking 

 both of these essential vitamins by mutually supplying each other. 



Spore lethals are relatively common in Saccharomyces. They 

 apparently are variations of the haplophase which do not affect the 

 viability of the haplophase, but which render the spore incapable 

 of germination. 



BIOCHEMICAL AND PHYSIOLOGICAL CHARACTERS 



In a sense all characters are either biochemical or physiological 

 but the term is applied to specific fermentative and synthetic abili- 

 ties. Such characters have an important advantage, namely, they 

 can be superimposed in one individual without complicating geneti- 

 cal analysis. For this reason we have concentrated on genes dif- 

 ferentiating stocks with regard to the fermentation of carbohydrates 

 and the synthesis of vitamins, amino acids, and nucleic acid com- 

 ponents. These single gene differences, however, have not distin- 



