112 Outline of Genetics 



The presence and absence hypothesis has been very generally 

 accepted because of its value in simpHfying our concepts and pro- 

 viding the most convenient scheme of notation. At the same time 

 it is recognized that this hypothesis may not strictly represent the 

 truth in all cases. There are two outstanding types of evidence 

 that can hardly be interpreted by means of the presence and 

 absence concept. One of these will be discussed later in another 

 connection (p. 151). The other is as follows. 



In some cases other possibihties may be realized in connection 

 with a single locus than merely the presence or absence of a given 

 gene. For example, at a given locus on one of the chromosomes in 

 corn, a condition W may exist, which results in colorless pericarp. 

 In other plants that same locus may bear the gene F, variegated 

 pericarp, and in still others S, completely colored pericarp. S is 

 dominant to V and V to W (see also p. 119). As a matter of fact, 

 these three are simply representatives of a series of ten different 

 conditions that may be present at a given locus. Such cases are 

 spoken of as systems or series of multiple allelomorphs, and it would 

 be difficult to harmonize them strictly with the presence and 

 absence hypothesis. 



The relation of systems of multiple allelomorphs to mu- 

 tation is significant. It would be possible to arrange the genes 

 involved in any system of multiple allelomorphs in a series, 

 placing at the top the one which was dominant to all the 

 others, and at the bottom the one that was recessive to all the 

 others. This series, however, in no way reflects the order in which 

 such genes have originated by mutation. In the fruit fly, there 

 is a famous series of multiple allelomorphs for eye color, ranging 

 from white through progressive steps in intensity of coloration; 

 but it is not true that white first mutated to a light shade, which 

 later mutated to the next darker shade, and so on; nor is it true 

 that this series of mutants came off in a regular sequence down the 

 scale of color intensity. In short, within a series of multiple 

 allelomorphs the mutants come off in discontinuous rather than 

 continuous series. It follows that mutations cannot be "led 

 along" in a given direction by means of selection. 



Further, although it is common that all the genes in a series of 

 multiple allelomorphs affect the same general character, exceptions 



