346 PRINCIPLES OF EMBRYOLOGY 



immediate effect, or whether a single gene can do different things. It is 

 clear there is no evidence for more than one type of gene-action in cases 

 where the whole of a complex pattern of manifestation can be shown to 

 consist of a set of secondary consequences of some single primary effect 

 (as in the mutation of 3 above, or Griineberg's rat lethal described on p. 

 342). Griineberg speaks of this as 'spurious pleiotropy', contrasting it 

 with 'genuine pleiotropy', in which the same gene would be doing 

 different things in different tissues. The difficulty with this latter category, 

 however, is that at present we have no certain way of detecting it. We 

 can discover cases which seem to be of the type produced by a mutation 

 of gene i, i.e. in which there are two or more apparently primary effects 

 which are not influenced by other tissues. But there is no way of telhng 

 whether the mutated gene i is doing the same thing in organ i as it is in 

 organ iii, and we can at best class this as an 'apparently genuine pleiotropy'. 



Most attention, from the embryological side, has recently been given 

 to cases of spurious pleiotropy, since the secondary effects may reveal 

 some of the epigenetic interrelations between tissues. 



The primary effects, however, are also not without interest. In a case 

 of 'apparently genuine pleiotropy' we are confronted with two or more 

 at first sight disconnected developmental processes, which are shown 

 actually to be related by the fact that one particular gene is involved in all 

 of them. They must therefore have some fundamental similarities ; and 

 the nature of these offers a very important problem which may go to 

 the very heart of the epigenetic systems involved. To take an example. 

 In Drosophila, split causes both an extra division in the bristle-forming 

 cells (followed by various abnormalities in their arrangement), and also 

 an effect on the facet-forming cells of the eye which is in the main a 

 deficiency in the normal number of cell divisions (accompanied also by 

 some irregularity in arrangement). Now the facet-effect o£ split is almost 

 exactly mimicked by morula, another effect of which is to reduce the 

 growth rate of the bristle-producing cells so that the chaetae are smaller 

 than normal. This bristle-effect of morula, in turn, is mimicked by spine- 

 less; and many of the alleles at the spineless locus cause as well the 'aristo- 

 pedia' phenotype, characterised by the conversion of the arista into a leg, 

 the basic alteration being perhaps one on the growth rate and folding 

 of the imaginal bud, We have then a series of developmental reactions, 

 revealed by the mutant phenotypes of split, morula, and aristopedia, which 

 would seem quite disconnected from one another were it not that the 

 overlapping pleiotropies show that there are some basic relationships at 

 the level of apparently prhnary gene action (Lees and Waddington 1942, 

 Waddington and Pilkington 1943). 



