. PHENOMENA OF INHERITANCE 281 



offspring are intermediate between their 

 parents owing to incompleteness of dominance, 

 rather than to incompleteness of segregation; 

 in such cases the parental types reappear in 

 the F 2 generation as in the cross between red 

 and white four o'clocks. Sometimes depar- 

 tures from the 3 to 1 ratio are caused by the 

 fact that two or more factors of the same sort 

 are involved in the production of a single char- 

 acter. Nilsson-Ehle found that when oats 

 with black glumes were crossed with varieties 

 having white glumes the ratio of 3 white to 

 1 black was usually found in the second gen- 

 eration; but one variety of black oats when 

 crossed with white gave in the second gener- 

 ation approximately 15 blacks to 1 white 

 which is the dihybrid ratio. From this and 

 other evidence he concludes that in this va- 

 riety of oats two hereditarily separable fac- 

 tors are involved in the production of black. 

 In crosses between red-grained and white- 

 grained wheat he usually got in the second 

 generation the monohybrid ratio of 3 red to 1 

 white, but three strains gave the dihybrid ratio 

 of 15 to 1 and two gave the trihybrid ratio of 



