Nos. 618-619] SHORTER ARTICLES AND DISCUSSION 361 



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F% "Zygotes Phenotypic Ratio t 



As Davenport clearly points out in the matter of black skin- 

 pigment, each gene finds its definite somatic expression regard- 

 less of the presence of other genes. Since the two genes work 

 out into the same somatic trait {%. e., are duplicate genes) which 

 somatic expressions differ only in quantity, they might well be 

 be called cumulative genes. The accompanying diagram (Ex- 

 periment Illustrative of Black Skin-Pigment Inheritance) gives 

 specific directions for running this experiment in a manner very 

 closely paralleling what Davenport found in nature. 



This experiment illustrates only one case in a long possible 

 series of modifications of the foundation F 2 di-hybrid phenotypic 

 blending ratio 1:2:1:2:4:2:1:2 :1. 



C. Other F 2 Di-Hybrid Series 



In a case in which gene "A" does not blend with its allelomor- 

 phic mate "a" in the F x soma, but presents a typical case of 



