l86 AN INTRODUCTION TO MODERN GENETICS 



which lie far along the horizontal part of the curve (Haldane);^ (3) push- 

 ing down the horizontal part of the curve, thus preventing large doses 

 from having too great an effect and making the flattening out occur 

 earlier (Muller).^ (Cf. p. 297.) 



The "effects" in the dose-effect curves mentioned above are the 

 limiting, asymptotic values of the processes caused by various doses 

 of genes. Goldschmidt^ has given a theory of dominance in which the 

 gene reactions are supposed to proceed at a constant speed not tending 

 to any Hmiting value. But the same principle of explanation is intro- 

 duced in another form: the dominance is explained by supposing that 

 what we actually observe is not the gene reaction itself but another 

 reaction which, when plotted against the dose, gives a curve which 

 rises steeply and then becomes horizontal. Goldschmidt supposes that 

 this second process goes on more slowly as time passes, and that a 

 given dose of the gene corresponds to a constant interval of time 

 during which the reaction is allowed to proceed : thus unit of dose is 

 the same thing as unit of time, so that if the visible reaction when 

 plotted against time gives an exponential curve, so must it when 

 plotted against dose. In this scheme the gene-reactions, which now 

 merely control the length of time for which the second visible reaction 

 proceeds, have become completely hypothetical. 



5. Mimic Genes 



In a species, certain types of morphological variation may occur 

 particularly frequently, and from diverse genetical and environmental 

 causes. In terms of the "geological" model of gene reactions, we could 

 say that the landscape defined by the genotype includes certain definite 

 valleys branching downwards, so that any sHght variation in the upper 

 part of the main valley may divert the gene reaction into one or other 

 of these already existing side channels. A very good example of this 

 type of behaviour is the Minute group of genes in D. melanogaster} The 

 Minutes are all dominant (and lethal when homozygous), and they all 

 shorten the bristles to various extents and have a characteristic effect 

 on the eyes, wings, etc. Their loci are scattered throughout the chromo- 

 somes, but although they are definitely different genes, they all produce 

 the same syndrome of effects, though with different strengths. Schultz 

 showed that in compounds (double heterozygotes) two Minutes do not 

 reinforce one another, and thus do not perform the same primary 



1 Haldane 1930, 19326, cf. Wright 1929. - Muller 19326. 



^ Goldschmidt 1932. * Schultz 1929. 



