THE ROLE OF GENES IN THE EPIGENETIC SYSTEM 333 



interpret his results in terms of the same sequence of developmental 

 mechanisms as the genetic study had revealed. Goldschmidt (1935^2) 

 Henke (1947), Schatz (195 1) and others have treated Drosophila pupae 

 v^ith just sub-lethal temperatures at definite times, and found that a series 

 of abnormalities of wing development occur which parallel in a striking 

 manner the forms produced by mutant genes. Such abnormalities which 

 are similar to the phenotypes characterising genetic races but which are 

 produced by environmental stimuh, are known as 'phenocopies . Henke 

 found the interesting fact that the time at which a temperature shock was 

 capable of causing the appearance of a phenocopy of some particular 

 mutant type was usually near to, or just before, the time at which the 

 development of that type first diverges visible from the normal. For 

 instance, shocks just before or during the contraction phase tend to pro- 

 duce dumpy- or hlade-\iik.Q. phenocopies. For each type of phenocopy there 

 is a 'sensitive period' during which it can be relatively easily induced^ 

 (Fig. 15.2). 



The fact that the sensitive period for the phenocopy of a gene occurs 

 very near the time at which the development of the mutant becomes 

 recognisably abnormal suggests at first sight that tliis is the stage of 

 development at which the gene becomes active. It is, however, by no 

 means clear just what might be meant by 'the time of action of a gene'. 

 If an embryo contains a certain mutant gene, that gene is present in its 

 cells from the time of fertilisation onwards. Long before there is any 

 visible abnormality of development, the gene may have been producing 

 some unusual substance which is merely stored up within the cell without 

 being detectable by existing methods. The fact that an external agent 

 can produce a given phenocopy most easily at a certain stage of develop- 

 ment is a sign that the relevant epigenetic process is most unstable at that 

 time. It is not surprising that it should be just at this time that the mutant 

 gene also begins to produce detectable divergences from the normal 

 course of development, but this does not tell us whether the gene has been 

 active earlier or not. Nor, of course, are we justified in concluding that 

 the gene acts by a mechanism similar to that of the environmental stimu- 

 lus; in fact during a sensitive period many different external stimuli (heat, 

 cold, x-rays, etc.) may produce similar effects, and it is clearly impossible 

 that all of them can be disturbing the system in the same way that the 

 gene does. Phenocopy studies can thus provide some information about 

 the stability of the epigenetic situation in the developing tissues, and can 



^ Occasionally there are two different sensitive periods, for instance when very similar 

 end-results can be brought about in two different ways. 



