156 MULTIPLE ALLELOMORPHS 



the new factor must act as an allelomorph to the first 

 mutant; as well as to the ''parent" normal allelo- 

 morph. 



Since these factors have the same location they 

 must all give the same linkage values with other fac- 

 tors. This has been shown to be true. For instance, 

 the factor for white eye color of Drosophila is very 

 closely linked to that for yellow body color. The 

 .''distance" between them is 1 unit, which means 

 that crossing over takes place about once in a 

 hundred times. Eosin eye color gives the same 

 crossing over frequency with yellow. 



White eye color gives with miniature wings about 

 33 per cent, crossing over. Eosin gives the same 

 value with miniature. 



White gives 44 per cent, of crossing over with 

 bar eye. Eosin has the same value. Similar rela- 

 tions hold for all of the characters of the first group; 

 they all have the same linkage values for eosin that 

 they have for white. This example indicates that 

 the conception of allelomorphs should not be limited 

 to two different factors that occupy identical loci 

 in homologous chromosomes, but that there may be 

 three, as above, or even more different factors that 

 stand in such a relation to each other. Since they 

 lie in identical loci they are mutually exclusive, and 

 therefore no more than two can occur in the same 

 animal at the same time. This is both demonstrated 

 by the facts and postulated by the chromosomal 

 mechanism. 



On a priori grounds also it is reasonable to suppose 



