156 AN INTRODUCTION TO MODERN GENETICS 



The concepts of genotype and phenotype are defined in the first 

 place in relation to differences between whole organisms. They are 

 not adequate or appropriate for the consideration of the development 

 of differences within a single organism. In this connection we do, 

 indeed, require the concept of the hereditary (chromosomal) constitu- 

 tion of the zygote, and we can without danger extend the meaning of 

 the word genotype to cover this. But the difference between an eye 

 and a nose, for instance, is clearly neither genotypic nor phenotypic. 

 It is due, as we have seen, to the different sets of developmental pro- 

 cesses which have occurred in the two masses of tissue; and these 

 again can be traced back to local interactions between the various genes 

 of the genotype and the already differentiated regions of the cytoplasm 

 in the egg. One might say that the set of organizers and organizing 

 relations to which a certain piece of tissue will be subject during 

 development make up its "epigenetic constitution" or "epigenotype" ; 

 then the appearance of a particular organ is the product of the genotype 

 and the epigenotype, reacting with the external environment. In 

 transplantation experiments, such as those described above, it is the 

 epigenotype which is altered. 



10. Substance Genes and Pattern Genes 



In discussing the effects of genes during development we must 

 attempt to make hypotheses as to their actions which fit in with the 

 general scheme of developmental mechanics which has just been given. 

 We can start by making a distinction between genes which apparently 

 affect the kinds or amounts of substances produced, and genes which 

 affect the patterns in which the substances are arranged. This distinc- 

 tion, like the similar distinction between evocation and individuation in 

 embryology, is not an absolute one. The "substances" with which we 

 are concerned may not be weU-defined chemical units but may in some 

 cases be tissues, which of course themselves have a pattern on a small 

 scale. One of the main problems of developmental physiology, how- 

 ever, is to discover the nature and origin of the comparatively large- 

 scale patterns found in organisms, and to try to understand, for in- 

 stance, how the characteristic arrangement of tissues in a limb can be 

 explained in terms of physico-chemical entities which have a pattern 

 on an altogether smaller scale. The category of pattern genes is intro- 

 duced because it makes it impossible to forget this most fundamental 

 problem. We shall class genes as pattern genes when they affect some 

 pattern of a supra-molecular order, and usually quite large in relation 



