M PHYSIOLOGICAL GENETICS 



reaction, were cases of pigmentation in guinea pigs or Drosophila 

 eyes. Numerous similar examples are found in genetic literature 

 (sec Stern, monograph, L930), which all lend themselves to the 

 same type of interpretation. 



The next point to be considered is the action of pleiotropic 

 genes in such series. One and the same gene, in different multiple 

 allelomorphic conditions, acts upon many phenotypic characters, 

 and this may be done by changing the rate of one chain of 

 reactions which affects a developmental process, thus leading 

 automatically to manifold effects (see foregoing chapter). In 

 this case, the different characters affected ought to show corre- 

 sponding series of gradations, provided this is possible. (This 

 restriction means that the development of an individual trait 

 might be of an all-or-none type, which would exclude inter- 

 mediate steps.) It must be kept in mind, however, that such 

 an expected parallelism need not be of the simple type of simul- 

 taneous and parallel change of each character. Let us take an 

 example. A series of effects may be produced by an earlier and 

 earlier onset of some process, as in the vestigial case. As 

 embryonic development consists of a series of determinative 

 processes which narrow down the potentialities of parts of the 

 embryo, an earlier onset of a change will affect more potential- 

 ities. If we take the vestigial case, it is a fact that the imaginal 

 disk for wing formation contains also a thoracic bud and that 

 these tw r o areas are separated only during development. Let us 

 assume that a series of earlier and earlier changes occur in the 

 wing Anlage in the imaginal disk. They will affect only the wing 

 down to the stage at which determination of wing and thorax 

 Anlage occurs. At an even earlier stage, however, the still 

 equipotential Anlage is affected, and the resulting change occurs 

 in both wing and thorax. The latter effect then will appear in the 

 series only near its highest point. This example shows that in a 

 given case the same reaction might lead to effects in different 

 organs which might begin at different levels of the series. It will 

 be easy to imagine manifold variations of this one example. 

 The following diagram represents three such possibilities of 

 pleiotropic effects upon the three characters ABC, caused by a 

 series of allelomorphs n 1 , n 2 , etc., all resulting from one and the 

 same reaction, which hits different embryological processes at 

 different levels. 



