THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 141 



typically different. Of the nine purples, only one breeds true in F 3 , 

 and of the remaining eight purples, four give families which segregate 

 in the ratio of 3 purple : 1 white, and four give families showing segrega- 

 tion in the ratio of 9 purple : 7 white. All the whites, although of different 

 genotypes, produce entirely white families. All these relationships are 

 shown clearly in the checkerboard. 



In Drosophila a large number of similar cases of like somatic effect 

 have been found to be dependent upon different factors. Here the 

 linkage values of the different factors with other factors have been 

 determined very precisely, and moreover the mutants have for the most 

 part arisen directly from the cultures, so that the relationships have 

 been established much better than in any other form. 



CP 



Cp 



cP 



cp 



CP 



Cp 



cP 



cp 



FIG. 64. Checkerboard of Ft of cross white (ccPP) X white (CCpp) maize, showing 

 phenotypes and F 3 segregation as well as genotypes. 



For body color at least three similar mutant factors result in almost 

 identical darker forms. The first of these to be discovered was the black 

 factor which is located in the second group of factors. The factor for 

 ebony body color is in the third group, and sable is a sex-linked factor. 

 Although so nearly alike that a mixed population could not be certainly 

 classified these particular races do show slight differences in coloration. 

 Similarly nearly identical results are obtained from three different 

 jaunty factors which cause the wings to turn up at the ends. Morgan 

 has also pointed out other such similarities in effect of different factors 

 which affect eye and wing characters, color, etc. 



Sometimes a dominant and a recessive factor give identical pheno- 

 typic results. For an illustration of this we may again turn to aleurone 



