THE GENETIC CONTROL OF PATTERN I93 



the whole pattern of the animal. During development new substances 

 and tissues are produced by the interaction of different regions of the 

 egg, and in this way the pattern gradually becomes more complex. If 

 the nature of the reacting substances is altered by a gene substitution, 

 the equilibrium which they attain will be altered and a new pattern 

 produced. 



As we have seen (p. 143), the case of Limnea gives some support to 

 the hypothesis that the pattern-properties of the egg cytoplasm are 

 themselves dependent on the genes of the mother in which the egg was 

 formed. 



2. Genes Affecting the Manifestation of Patterns 



Some genes alter the patterns which can be seen in the adult organ- 

 ism, but on analysis are found merely to have changed the expression 

 of a pattern which itself remains fundamentally the same. The simplest 

 case is when a gene alters the type of pigment which is distributed in a 

 pattern. The fact that the actual pattern is not altered by changing the 

 colour of the pigment is so obvious that we should hardly be tempted 

 to call such a gene as a pattern gene at all. But other cases are rather 

 more subtle. For instance, many colour patterns are based on a funda- 

 mental pattern of readiness to receive pigment during a certain period 

 of development. If a gene causes an increased amount of pigment to be 

 formed, the size of the pigmented patches may be increased. Gold- 

 schmidt^ has explained some melanotic forms in Lepidoptera in this 

 way. Another possible mechanism, to which Goldschmidt has also 

 drawn attention, is as follows. Suppose that the deposition of pigment 

 is dependent on the diffusion from a centre of a pigment precursor, 

 which can be converted into pigment only during a certain limited 

 period of development; then if a gene causes the precursor to be 

 formed earlier than usual, the diffusion will cover a larger area before 

 the pigment-forming reaction is brought to an end, and the coloured 

 areas will be larger. In the Lepidopteran cases described by Gold- 

 schmidt, the precursor substance which diffuses out from the centre of 

 the future spot seems actually to be something which slows up develop- 

 ment of the scales. Pigment formation happens at a later period, and 

 can only occur in the relatively undeveloped scales affected by the 

 diffusing substance, the rest of the scales being too advanced to be 

 coloured. 



One of the most fully analysed cases of pattern formation in the 

 ^ Goldschmidt 1927. 



