HEREDITY 



595 



which are unable to carry on the normal 

 chemical activities of the wild strain. In- 

 vestigators hypothesized that if the succes- 

 sive steps, chemical reactions, in these syn- 

 theses were controlled by a series of genes, 

 and if one or more mutations were induced 

 in the genes of this organism, that the re- 

 quirements for its growth would change. 

 Wild type Neurospora was irradiated in the 

 hope of inducing mutations. Mutations were 

 produced and mutant forms were found 

 that require the addition of a particular 

 vitamin, amino acid, purine, or pyrimidine, 

 to the ordinary food before growth can oc- 

 cur, showing that the mutants have lost the 

 ability to synthesize one of these substances. 

 Loss of this capacity to synthesize vital sub- 

 stances would be lethal in nature. In every 

 case, the loss of synthetic power has been 

 found to be inherited. 



Closer study of the mutants has shown 

 that one gene is frequently responsible for 

 one reaction in the synthesis of a given 

 product. Thus one gene may control one 

 of the chemical processes involved in the 

 synthesis of niacin, another gene one of the 

 processes in the synthesis of thiamine, and 

 so on. One of the interesting results of these 

 mold investigations is that the metabolic 

 processes so far discovered are closely simi- 

 lar to, if not identical with, those of animals. 

 This indicates that the findings made with 

 this organism will probably be applicable to 

 living things in general. 



In some cases it seems that the relation- 

 ship between genes and chemical reactions 

 is more complex than in the examples given 

 above, but there is no doubt that the kinds 

 of chemical reactions which occur in a cell 

 are controlled by the genes in its nucleus. 

 The way in which genes exert this control is 

 by controlling the production of the en- 

 zymes which catalyze the chemical reactions 

 in question. Thus, for example, there is a 

 mutant in Neurospora which fails to pro- 

 duce the enzyme tr}'ptophane synthetase, 

 and which is, therefore, incapable of syn- 

 thesizing the amino acid tryptophane. 



Several effects from one gene 



Most genes, if not all, probably produce 

 their effects by controlling one or another 

 chemical reaction in the developing organ- 

 ism. Even if a gene controls just a single 

 chemical reaction, the product of that reac- 

 tion could be utilized in several different 

 subsequent reactions. The amino acid trypto- 

 phane, for example, would be incorporated 

 into many different proteins. Thus a 

 single gene can have multiple effects even 

 if it controls but a single chemical reac- 

 tion. 



An example is the gene-producing vestigial 

 wing in Drosophila (Fig. 440). This gene 

 not only reduces wing size, but also changes 

 the positions of some of the body bristles, 

 slows the growth rate, and shortens the 

 length of life. Another example is a form of 

 hereditary feeble-mindedness in man, which 

 is always accompanied by the excretion in 

 the urine of phenylpyruvic acid, a sub- 

 stance that a normal person oxidizes com- 

 pletely in his body. In fact, most genes seem 

 to have multiple effects on the embryonic 

 development of organisms. 



Selection 



Of interest, especially to agriculture, is 

 the light that the study of genetics throws 

 on the subject of selection. Man has for 

 many centuries attempted to improve the 

 traits of domesticated animals and plants 

 by selecting for breeding purposes superior 

 individuals. This has been effective to a cer- 

 tain extent, since, by this method, the 

 progeny received the genes for the desired 

 traits. However, the progeny also received 

 the genes for traits (recessives) that did not 

 appear in the parents. Also many of the 

 traits selected were due to environmental in- 

 fluences and were not inherited. It seems 

 evident that selection is effective only when 

 heritable traits are involved and that more 

 rapid progress is possible if the genotypic 

 constitution of the parents is knovm. 



