178 MULTIPLE FACTORS 



the next generation when the plant of that composi- 

 tion is self -fertilized. It will be observed that the 



1 :0 ratio is expected 7 times. 

 3 : 1 ratio is expected 4 times. 

 15 : 1 ratio is expected 4 times. 

 : 1 ratio is expected 1 time. 



This test was applied by Shull to his Fo plants of the 

 triangular type. There were seven families that 

 gave a 1 : ratio, four that gave approximately a 3 : 1 

 ratio, and six that gave a 15 :1 ratio. These results 

 are in fair accord wuth the expected numbers given 

 above. 



When a further test was carried out by breeding 

 from the six 15:1 families of the F3 group above 

 (which should be expected to give the same results 

 as the F2 class, because they have the same composi- 

 tion), the ratios obtained were as follows: 



1 :0 ratio expected 35; realized 39. 



3 :1 ratio expected 20; realized 12. 



15 :1 ratio expected 20; realized 26. 



The results agree again fairly well with the expecta- 

 tion. 



A second test is found in self-fertilizing plants from 

 families that gave a 3 : 1 ratio. As the diagram shows 

 these contain only the one (^'c") or the other (^^d") 

 factor, they should give only homogeneous families 

 and 3 : 1 families — never 15:1 families. This result 

 also was obtained. 



Nilsson-Ehle found that three recessive factors 

 must combine to produce an effect which, in the 



