ABSTRACT
THE SCIENCE of genetics has clearly shown that when an animal differs from nearly related forms, the nature of these differences is nearly always controlled by genes carried on its chromosomes. It is clear then that genes must be amongst the most important causal entities which playa 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, competence, etc. Genetics, following a quite different mode of analysis, formulates its answer to the same question in a quite different way. It finds 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 development. 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:
(2) At each step there are several genes acting, and the actual development which occurs is the resultant of a balance between the opposing gene-instigated tendencies.
