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THE AMERICAN NATURALIST [Vol. LII 



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Zygotes: Phenotypic Rat it 



Mendelian dominance, while gene "B" blends with, its mate 

 "b" in the F 1 soma, the di-hybrid F 2 ratio resulting from com- 

 bining two such trait-pairs is found by multiplying the F 2 mono- 

 hybrid dominant relation 3 + 1 by the F 2 mono-hybrid blending 

 relation 1 + 2 + 1, giving when expressed as a ratio the somatic 

 F 2 di-hybrid relation 3:6:3:1:2:1. Among other special cases 

 to be considered are those involving crossing-over, non-disjunc- 

 tion and sex-limited inheritance. 



When the genie and somatic specifications of a ratio are 

 known it is not difficult to contrive a chemical parallel for it. 

 The use of such experiments consists not only in clarifying the 

 conception of the particular situation at hand, but also in pro- 

 viding in the laboratory a nearer approach than is usually used 

 for demonstration to what is actually happening in heredity. The 

 eh.-iiii.-nl analogy between what happens in the zygote-holding test- 

 tube and what actually happens in the somatic working out of the 



