Til E MUTATED GENE 81 



strosities in vertebrates which we studied before. But it must 

 be kept in mind that this time element is not the only cause of 

 pleiotropic gene effects. If the direct action of a gene consists 

 in the production of a substance acting like a hormone, this may 

 affect development all over the body. If the primary effect is 

 of a strictly localized type, nothing else may be affected, except 

 perhaps readjustments in the growth of neighboring parts. 

 Many intermediate and similar cases may be worked out theo- 

 retically or be derived from the study of the embryology of 

 mutants. The details will differ in the various cases. But it 

 will always be found that the so-called pleiotropic effect of a gene 

 is not a property of the gene and not a general property of gene 

 action but an embryological consequence of the time, place, and 

 type of the primary disturbance of development by the mutant 

 gene. 



This explains also why the different characters controlled by a 

 mutant pleiotropic gene may show very different reactions. 

 An extreme case of this type has been reported by Eker (1935). 

 The recessive mutant short wing in Drosophila produces small 

 and rough eyes, short wings, scalloping, abnormal venation. 

 These characters have a strange relation to temperature: At 31°, 

 the viability is zero and increases with lower temperature. The 

 phenotypic expression of the traits is 100 per cent at 27.5° and 

 decreases to zero at 14°. (Of course, many cases are known of the 

 dependence of phenotypic expression upon -temperature or 

 moisture.) But in addition the different traits have a different 

 temperature-sensitive period — some during the whole of develop- 

 ment; others, only at a definite stage. (This case might, how- 

 ever, turn out not to be the result of a simple gene mutation.) 



B. Multiple Alleles 



The phenomenon of multiple allelomorphism, discovered by the 

 early Mendelian scholars, has always played an important role 

 in the study of gene action, since the Drosophila workers proved 

 that series of multiple allelomorphs are situated at the same locus 

 of the chromosome and therefore may be conceived as different 

 conditions of the same gene, whatever this may mean. As far 

 as I know, Wright (1916) and Goldschmidt (19166, 1917r/) were 

 the first to derive from the study of such a phenomenon general 

 ideas about the type of action of the gene. According to Wright, 



