THE ROLE OF GENES IN THE EPIGENETIC SYSTEM 



339 



The reactions leading to the brown pigment must be hnked in some way 

 with those leading to the red, since some genes, particularly those of the 

 white locus, affect them both. Probably this gene is essential in connection 

 with some common substrate or carrier-protein, which has to link up 

 with the m+ substance and with something equivalent in the reaction-series 

 leading to the red pigment. It is interesting to note that the cn+ substance 

 can only produce its effect during a certain limited period of development, 

 extending from sometime after puparium formation for about forty- 

 eight hours. Presumably before that time the kynurenine is not yet avail- 



BROWN PIGMENT 



f,cd+, 



CHROMOGEN 

 (cn* subsl-once) 



RED PIGMENT 



bw^QENE-- 



PRECURSOR 



a. 



C-CHiCHCOOH 

 NKt 



J^ (y* subsVanco.) 



Cx?- 



CHfCH-COOH 

 NHt 



'COMMON: 



SUBSTRATE, 

 CARRIER, 

 OP ENJYME 



*— W^OBNE 



CC-OXVTRYPTOPHAN 



TRYPTOPHAN 



Figure 15.4 

 Eye pigment development in Drosophiln. (After Beadle 1945.) 



able, while by the end of the period of sensitivity it must have been con- 

 verted into something else. This provides a clear parallel to the restriction 

 of periods of competence found in more usual experimental embryological 

 studies. It is also worth pointing out that phenocopies of eye-colour 

 mutations cannot be easily produced by environmental shocks of a general 

 nature, such as high or low temperatures; the epigenetic system is not 

 complex enough to be unstable with respect to such non-specific agents. 

 Another well-analysed system of pigmentation is that of the guinea- 

 pig hair colour (Wright 1942). It will not be described here but it well 

 illustrates the point which is being made, namely the real complexity of 

 apparently simple developmental processes. 



