176 AN INTRODUCTION TO MODERN GENETICS 



The chemical system is in this case probably very much simpler than 

 in plants, although the actual chemical nature of the pigments (which 

 are related to melanin) is still unknown. There seem to be only two 

 pigments involved, a yellow and a red, which are related to one another 

 as oxidation-reduction products. 



Two processes occur in the development of the eye-colour: the 

 synthesis of the pigment in the reduced (yellow) form, and its oxida- 

 tion to the red form. Only in very few mutants is one of the pigments 

 completely absent (no red in sepia, no yellow in vermilion), but all 

 variations are found both in the total quantity of pigment, the time of 

 its formation and the proportion of it which becomes oxidized. Schultz 

 divides the genes into two groups: those in which pigment formation 

 begins at the same time as in the wild-type, but follows a different 



Fig. 83. The Anthocyanin Molecule. — ^There 

 is always a sugar residue at 3. Genes are known 

 with the following effects: (1) Oxidation at 3'; 



(2) Oxidation at 5' when 3' is already oxidized; 



(3) Oxidation at both 3' and 5'; (4) Methylation 

 of the hydroxy! at 3'; (5) Methylation of hy- 

 droxyls at 3' and 5'; (6) Substitution of a sugar 

 at 5; (7) Acylation with an organic acid residue 

 (position uncertain). 



course either in the amount of pigment formed or in the amount 

 oxidized; and those in which the early stage of pigment formation is 

 suppressed. Some of the genes concerned are, as we know, related as 

 hyper-hypo-morphs ; the double heterozygote shows an intermediate 

 condition between the two homozygotes. These genes are mostly 

 allelomorphic to one another. In combinations of non-allelomorphs, 

 there is a difference according to whether or not the genes belong to 

 the same group in Schultz's classification.^ Those within one group seem 

 to affect various elements in the complex chemical system leading to 

 one of the two reactions, pigment-formation or pigment oxidation, and 

 usually the gene with the most marked effect, i.e. the one giving the 

 brighter coloured eye, is epistatic; presumably the part of the reaaion 

 it controls has become the limiting factor. In combinations between 

 groups, the genes affect different systems and their effects are sum- 

 mated; early pigment formation is suppressed by the gene of one group, 

 and when it does start the kind of pigment synthesized is governed by 

 the gene of the other group. 



The interaction of eye-colour genes has also been studied by bringing 

 tissues containing different genes into physiological connection. It was 



^ Cf. Mainx 1937. 



