362 PRINCIPLES OF EMBRYOLOGY 



There is a considerable body of evidence from genetics which, although 

 it by no means settles the question, tends to suggest that all genes normally 

 remain in being in all types of differentiated cells. 



It is, of course, common to fmd that a given mutant gene produces a 

 rather locahsed abnormality and appears to be inactive elsewhere, and if 

 this evidence were taken at its face value, there would be nothing to 

 prevent our supposing that the gene had been completely inactivated or 

 lost in those regions in which it has no visible effect. However, Wadding- 

 ton (1953) has shown that several genes in Drosophila are actually in 

 operation in regions in which their influence is not obvious at first sight. 

 For example the well-known gene vestigial causes a severe reduction in 

 the size of the wings, but seems to have no influence on the immediately 

 neighbouring thorax. But in flies which are homozygous both for 

 vestigial and a gene such as dachsous, wliich does affect the thorax, it can 

 clearly be seen that vestigial is active not only in the wings but also in the 

 body (Fig. 16.8). One must assume that the effect of the vestigial gene on 

 the thorax normally falls below some threshold and produces no visibly 

 abnormal result unless the development of the thorax has already been 

 upset, and its canahsation weakened, by the action of some other mutant 

 such as dachsous. There are certainly many other cases of such sub- 

 threshold effects, as would be expected if all genes are effective, to a 



Figure 16.8 



(On the left). The gene dachsous causes a slight enlargement of the thorax 

 o{ Drosophila; vestigial reduces the wings to vestiges, and apterous has a still 

 more severe effect of a similar kind. In the fly shown, which is homozygous 

 for all three genes, it is clear that vestigial and/or apterous have sub-threshold 

 effects on the thorax which become effective in the combination with 



dachsous. (From Waddington 1953.) 

 (On the right.) Sections through the spermathecae of various mutants, 

 showing how the shape is affected by genes whose main action is else- 

 where. (From Dobzhansky 1927.) 



