436 THE POPULAR SCIENCE MONTHLY 



Indeed, the evidence for the individuality 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 inheri- 

 tance than that discovered by Mendel. Are there cases which do not 

 segregate according to Mendelian expectation ? 



Chromosomes 



XX parents 



d 9 



x © en „ t 



l X I n Gametes 

 X X ?J 



9 d 



2£ X ? 



y | + ( Gametes 



X 6 



& S> &<S> M XX » X 



9 ? d d 9 9 d d 



Fig. ti4. Diagram op Inheritance of Color-blindness through Female. A 

 color-blind female transmits her defect to all her sons, to half of her granddaughters 

 and to half of her grandsons. Corresponding distribution of sex chromosomes on right. 

 (After Morgan. ) 



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 parental types in the F 2 generation or in which 

 the ratio for a monohybrid differed from the well known 3:1 ratio, 

 were supposed to be non-Mendelian. 



However, further work has shown that some of these are really Men- 

 delian. 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 genera- 

 tion as in the cross between red and white four-oYlocks. Sometimes 

 departures from the 3: 1 ratio are caused by the fact that two or more 

 factors of the same sort arc involved in the production of a single char- 

 acter. Nilsson-Ehle found that when oats with black glumes were 



