SCOPE OF GENETICS 29 



Our purple plant is thus cross-bred for 

 four factors, containing only one dose of 

 each. 



We let it fertilise itself, and its offspring 

 show all the possible combinations of the 

 four different factors and their absences which 

 the genetic constitution of the plant can 

 make. 



Note that one of the combinations we 

 expect to find is missing. There are white 

 erect and white hooded white because they 

 are lacking one or other of the comple- 

 mentary ingredients necessary to the pro- 

 duction of pigment. There are purple erect 

 and purple hooded, of which the purple erect 

 must perforce contain all the four factors, 

 and the purple hooded must similarly con- 

 tain all of them except that for erectness. 

 But when we turn to the red class we are 

 surprised to find that they are all erect, none 



