284 



EVOLUTION AND GENETICS 



combinations from the two parents result in the individuals of 

 the F2 generation. Among these sixty-four the eight phenotypcs ex- 

 press the trihybrid ratio 27:9:9:9:3:3:3:1. The diagram shows 

 both the phenotype and genotype of each individual and illustrates 

 the rapid increase in complexity of the F2 generation as the number 



TSY 



Male gametes 

 TSy TsY Tsy tSY tSy 



tsY tsy 



TSY 



TSy 



TsY 



to 



s 



Tsy 



"i tSY 





tSy 



tsY 



tsy 



Fig. 163. — Diagram to illustrate the F> generation of a hybrid between 

 tall smooth yellow and dwarf wrinkled green peas. Ratio of phenotypes 

 27:9:9:9:3:3:3:1. 



of characters is increased. Any further examples would merely 

 repeat the same process without adding to the facts already 

 disclosed. 



Theoretical Calculations. We have seen that a single pair of 

 allelomorphs make possible four combinations according to the 

 law of chance, that two make possible sixteen combinations, and 

 three sixty-four combinations. It is therefore apparent that the 

 number of possible combinations is equal to four raised to a 

 power indicated by the number of different characters involved, 



