280 HEREDITY AND ENVIRONMENT 



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 segregate ac- 

 cording 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 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 



