SCOPE OF GENETICS 27 



a purple flower with an erect standard. The 

 colour comes from the concurrence of com- 

 plementary elements. A dose of a certain 

 ingredient from one parent meets a dose of 

 another ingredient from the other parent 

 and the two make pigment in the flower. 

 From other experiments we know that the 

 purple colour of the pigment is due to a 

 dose of a third ingredient brought in from 

 the hooded parent ; and that in the absence 

 of that blue factor, as we may call it, the 

 flower would be red. The standard is erect 

 because it contains a dose of the erectness- 

 factor from the erect parent, and the hooded 

 parent can readily be proved to owe its 

 peculiar shape to the absence of that 

 element. 



Our purple plant is thus cross-bred for 

 four factors, containing only one dose of 



each. 



We let it fertilise itself, and its oflspring 



