CHAPTER XV 



THE ROLE OF GENES IN THE EPIGENETIC SYSTEM 



THE SCIENCE of gciietics has clearly shown that when an animal differs from 

 nearly related forms, the nature of these differences is nearly always con- 

 trolled by genes carried on its chromosomes. It is clear then that genes must 

 be amongst the most important causal entities which play a role in guiding 

 development. We have so far discussed the question of why an organ, 

 such as a limb, develops as it does in terms such as organisers, fields, com- 

 petence, etc. Genetics, following a quite different mode of analysis, formu- 

 lates its answer to the same question in a quite different way. It fmds that 

 the development of the organ is dependent on the activities of certain 

 genes in the fertilised egg. The task of this chapter is to present the picture 

 of the development of an organ or tissue as seen in terms of genes. This 

 will provide a view of the epigenetic system which we must take as being 

 complementary to that derived from experimental embryology. 

 I. Developmental pathways and their genetic control 



In using the data of genetics to throw light on the general character of 

 developmental processes we are not concerned with the way in which 

 any particular gene obtains its effect — a question we shall take up in the 

 next chapter. Here we want to start from the other end, taking an organ 

 or tissue and seeing how genetics would lead us to envisage its develop- 

 ment. It will be convenient before discussing particular cases to summarise 

 the general principles which we shall in fact fmd to emerge. The most 

 important of these are : 



(i) The development of an organ or complex substance takes place in a 

 series of steps, each of which is affected by genes. 



(2) At each step there are several genes acting, and the actual develop- 

 ment which occurs is the resultant of a balance between the opposing 

 gene-instigated tendencies. 



(3) At certain stages in the development of an organ, the system is in a 

 more than usually unstable condition, and slight disturbances at such times 

 may produce large effects on the later events. Such times have been called 

 'epigenetic crises'. 



(4) An organ or tissue is formed by a sequence of changes which can 

 be called the 'epigenetic path' leading to it. In a normal egg which con- 

 tains the genes usually found in the wild individuals of the species, these 

 paths are rather defmitely distinct from one another, so that a developing 



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