GENETICS 587 



cytoplasm of the cell, where it in turn impresses this specific surface con- 

 formation onto a protein molecule as it is synthesized and converts it 

 into the specific enzyme. 



If we assume that a specific gene may indeed produce a specific 

 enzyme by this or some other method, we must next inquire how the 

 presence or absence of this specific enzyme may affect the development 

 of the zygote. The expression of any trait is the result of a number of 

 chemical reactions which occur in series, with the product of each reac- 

 tion serving as the substrate for the next: A -^ B -^ C -^ D. The dark 

 color of most mammalian skin and fur is due to the pigment melanin 

 (D), produced from dihydroxyphenylalanine (dopa) (C), produced in 

 turn from tyrosine (B) and phenylalanine (A). Each of these reactions is 

 mediated by a particular enzyme; the conversion of dopa to melanin, 

 for example, is controlled by the enzyme dopa oxidase. The condition 

 known as albinism, characterized by the absence of melanin, results from 

 the absence of dopa oxidase. The gene for albinism, a, does not pro- 

 duce the enzyme dopa oxidase, but its normal allele, A, does. 



In most animals and plants it is difficult to investigate the stepwise 

 control of the expression of a character except those in which some 

 colored product is formed. This difficulty was overcome when George 

 Beadle and Edward Tatum conceived the idea of irradiating the bread 

 mold, Xeurospora, and looking for mutations which interfered in some 

 way with the normal reactions by which the chemicals essential for its 

 growth are produced. The normal bread mold requires as raw materials 

 only sugar, salts, inorganic nitrogen and biotin, the so-called "minimal" 

 medium (Fig. 33.6). By exposing the mold to x-rays or ultraviolet rays, 

 a great many mutations were produced. After irradiation the mold was 

 supplied with "complete" medium, an extract of yeast which contains 

 all the known amino acids, vitamins, and so on. Any nutritional mutant 

 produced by the irradiation will thus be enabled to survive and repro- 

 duce to be tested subsequently. 



A bit of the irradiated mold is then placed on minimal medium. If 

 it is unable to grow we know that a mutant has been produced which 

 interferes with the production of some compound essential for growth. 

 Then, by trial and error, by adding substances to the minimal medium 

 in groups or singly, the nature of this missing substance is determined. In 

 each instance genetic tests show that the mutant strain produced by 

 irradiation differs from the normal wild mold by a single gene, and 

 chemical tests show that if a single chemical substance is added to the 

 minimal medium the mutant strain can grow normally. The inference 

 is that each gene produces a single enzyme which regulates one step in 

 the biologic synthesis of this chemical substance. It has been possible 

 in some instances to show that the particular enzyme cannot be extracted 

 from cells of the mutant strain but can be extracted from those of 

 normal Xeurospora. The synthesis of each of these substances includes a 

 number of separate steps, each mediated by a gene-controlled enzyme. 

 An estimate of the minimal number of steps involved can be obtained 

 from the number of different mutants which interfere with its pro- 

 duction. 



