GENE CONTROLLED PROCESSES I75 



2. Genetic Control of the Kind of Substance Produced 



Goldschmidt has argued that the quantitative control exerted by 

 genes is a sign that different allelomorphs of the locus themselves differ 

 only in quantity. They may represent different amounts of an enzyme 

 catalyzing the reaction producing the substance, pigment or whatever 

 it may be. This conclusion may be true of some hypo-hyper-morphs. 

 But it is also possible to suppose that the genes differ in producing 

 different enzymes which catalyze the same reaction to different degrees 

 or even catalyze different reactions. In fact, in some cases there is no 

 doubt that the two allelomorphs do cause the production of different 

 mzymes or other chemical substances.^ Perhaps the clearest cases are 

 the blood-group genes of man. Two allelomorphs A and B each pro- 

 duce a specific isoagglutinogen quite independently of the presence of 

 the other allelomorph or of any other genes in the nucleus. The third 

 allelomorph at the locus produces no isoagglutinogen: it behaves as 

 an amorph to the other two which are neomorphs to it and to each 

 other. There is similar evidence of the production by a-, neo-morphs 

 of substances which are actually enzymes. A well-known example 

 is the recessive gene (amorph) which causes the loss of the enzyme 

 enabling man to oxidize homogentisic acid, which is therefore excreted 

 unchanged in the urine (a condition known as alkaptanuria). Similarly 

 there is an a-, neo-morphic gene pair in rabbits, in which the dominant 

 (normal, neomorphic) gene produces an enzyme which enables the 

 animal to oxidize any xanthophyll in its diet, a process which is impos- 

 sible to the mutant lacking this enzyme, so that its fat becomes coloured 

 yellow. 



The most fully studied example of genetic control of the kind of 

 substance elaborated is that on the anthocyanin pigments of flowers, 

 which we owe largely to Robinson and Scott-Moncrieff.^ The nucleus 

 of the anthocyanin molecule is affected by several kinds of substitutions, 

 which are controlled by specific genes, which presumably give rise to 

 substances, probably enzymes, which enable the various substitutions 

 to occur. Further genetic control of colour is obtained by (i) variations 

 of the P^ of the cell-sap, since many of the anthocyanins act as P^ 

 indicators, and (2) the synthesis of co-pigments (substances, them- 

 selves colourless, e.g. flavones and tannins, which bring about a change 

 of colour when added to solutions of anthocyanins). 



Schultz^ has studied the action of eye colour genes in Drosophila. 



^ Rev, Haldane I935«. " Scott-Moncrieff 1936, 1937 ^ Schultz 1935. 



