io8 Heredity and Environment 



develop in hybrids to the exclusion of their opposites whereas 

 under other conditions these results may be reversed or the char- 

 acters may be intermediate. The principle of dominance is not a 

 fundamental part of Mendelian inheritance. Even when the 

 characters of hybrids are intermediate between those of their 

 parents, if the parental types reappear in the F 2 generation we 

 may be certain that we are dealing with cases of Mendelian 

 inheritance. 



3. The Principle of Segregation. The individuality of inheri- 

 tance units, and their segregation or separation in the sex cells 

 and recombination in the zygote are fundamental principles of 

 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 2 generation or in which the ratio of dominants 

 to recessives differed from the well kmfwn 3 to I 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 



