Phenomena of Inheritance 107 



the Mendelian doctrine. Indeed the evidence for the individual- 

 ity and continuity of inheritance units is based entirely upon such 

 segregation and recombination, so that the entire Mendelian theory 

 may be said to rest upon the principle of segregation. If there are 

 cases in which such segregation does not take place they belong 

 to other forms of inheritance than the Mendelian; if segregation 

 occurs in every instance there is no other type of inheritance than 

 that discovered by Mendel. Are there cases which do not segre- 

 gate according to Mendelian expectation? 



When the Mendelian theory was new it was generally supposed 

 that there were forms of Inheritance which differed materially 

 from the Mendelian type ; indeed it was supposed that the latter 

 was one of the less common forms of heredity and that blending 

 of parental traits and not segregation was the rule. All cases in 

 which the characters of the parents appeared to blend in the 

 offspring or in which there was not a clear segregation of the par- 

 ental types in the F z generation or in which the ratio of dominants 

 to recessives differed from the well known 3 to 1 ratio were sup- 

 posed to be non-Mendelian. 



Unusual Ratios. — However further work has shown that most 

 of these cases are really Mendelian. Sometimes offspring are in- 

 termediate between their parents owing to incompleteness of domi- 

 nance, 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 departures 

 from the 3 to 1 ratio are caused by the fact that two or more fac- 

 tors of the same sort are involved in the production of a single 

 character. 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 generation ; but 

 one variety of black oats when crossed with white gave in the 

 second generation approximately 15 blacks to 1 white which is 

 the dihybrid ratio. From this and other evidence he concludes 

 that in this variety of oats two hereditarily separable factors are 

 involved in the production of black. In crosses between red- 



