PHENOMENA OF INHERITANCE 287 



there was not a clear segregation of the par- 

 ental types in the F 2 generation or in which 

 the ratio for a monohybrid differed from the 

 well known 3 to 1 ratio were supposed to be 

 non-Mendelian. 



However further work has shown that some 

 of these are really Mendelian. Sometimes 

 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- 



