GENES, MOLECULES AND PROCESSES 



excessive ability to destroy auxin by oxidation. On the other hand 

 the recessive alcaptonuric in man owes his pecuUarity to a character- 

 istic inabiUty to destroy honiogentisic acid by oxidation. Again the 

 Dahiiatiandog is distinguished from other breeds by a single recessive 

 gene, w^hich reduces the transformation of uric acid into allantoin, 

 and so increases the concentration of uric acid in the urine about ten 

 times. 



SERINE 

 HOCHjCHNHzCOOH' 



CHzCHNHjCOOH 



J 



i ANTHRANILIC i INDOLE TRYPTOPHANE 



ACID 

 ICENEll |gen"e2| 



Fig. 39. — The bio-synthesis of tryptophane proceeds in three stages in Neurospora 

 crassa which can be blocked by genes at two points at least. Mutation of Gene i 

 blocks the first stages, that of production of the first precursor, anthranilic acid. 

 Mutation of Gene 2 blocks the second stage, conversion of anthranilic acid into 

 indole. Thus Gene i is considered to govern the production of an enzyme catalyzing 

 the production of anthranilic acid, and Gene 2 the production of a second enzyme 

 catalyzing the conversion into indole. Where only indole or tryptophane was 

 recognizable, the genes would appear complementary in action, but where anthranilic 

 acid was also detectable Gene i would appear in its true light, as epistatic to Gene 2 

 (see Fig. 38) (based on Beadle, 1945). 



So much for single genes and simple processes. The combination 

 of genes and processes can best be seen in simple organisms such as 

 bacteria and fungi. By X-raying spores o£ Neurospora crassa Beadle, 

 Tatum, Horowitz and others have produced single-gene recessive 

 mutants, which differ from the wild type in their inability to grow, 

 or at least to grow well, on media lacking certain substances, amino- 

 acids, vitamins, pyrimidine bases and so on. Now the antennaless 

 fly seems unable to make efficient use of vitamin Bg, and therefore 

 requires it in greater quantity to produce anteimae. In the same 

 way these mutant moulds are unable to make use of precursors of 

 these various substances, and so require a supply of them ready- 

 made in their food. 



Different genes may, by mutation, lead to different kinds of 



162 



