THE ROLE OF GENES IN THE EPIGENETIC SYSTEM 



345 



would be changed except in organ i, where a primary manifestation would 

 occur. If 3 mutated, that would cause a primary effect in iii and a second- 

 ary one in I, in the reaction-chain of which an essential link is a product of 

 III, the 'hormone' b. If, before the lack of b had had irreversible conse- 

 quences, organ i were transplanted into another host with unmutated 3 , 

 then supplies of b would be available to it and it would develop normally. 

 Again, mutations in 4, 5 and 6 would cause secondary effects in i and m as 



1 11 I 



Figure 15.7 



Diagram of a sequence of gene reactions in three different tissues. See text. 

 (From Hadorn 1950.) 



well as a primary one in 11. But if a mutation occurred in genes i or 2 or 

 10, both organs i and m would be altered, and these alterations would 

 both be primary ones. 



When a gene affects several different organs or tissues it is said to be 

 'pleiotropic'. The term is not always a very precise one, since in border- 

 line cases there may be difficulty in deciding whether two effects are to be 

 considered the same or different (e.g. in Beadex or Lyra above). The con- 

 cept has, however, been widely discussed, since it clearly is of great im- 

 portance to discover whether any one gene can only exert one type of 



