MENDELISM AND OTHER METHODS OF DESCENT 211 



the expression tendencies of gametes the mathematical rehitions 

 of Mendelism can all be understood without the necessity of 

 predicating character-units or pure germ cells. 



Dominance, as a manifestation of expression-polarity, is not 

 a merely secondary matter in Mendelism, but affords a key to 

 the whole phenomenon, since it renders unnecessary the false 

 assumption that gamete formation is the reverse of fertilization. 



Interfolai- Inheritance. — In interpolar inheritance one of the 

 divergent characters is not dominant over the other, but the two 

 characters receive intermediate expression in all of the conju- 

 gates and in one half of the perjugates. 



This shows that the gametes formed by the intermediate con- 

 jugates represent the divergent parental characters in equal 

 numbers, as in Mendelism, but the lack of dominance leads to 

 intermediate expression of characters in the conjugate genera- 

 tion, and in one half of the perjugates and other succeeding 

 generations, since half of the gametes may be expected to mate 

 with others having the same polarity, and half with those hav- 

 ing a different polarity. 



Mosaic Inheritance. — Mosaic inheritance differs from inter- 

 polar inheritance in that the divergent characters are not com- 

 pletely averaged or blended, but pieced together, as it were, 

 after the manner of a mosaic. Thus in crosses between black 

 and white poultry, interpolar inheritance brings a conjugate 

 generation of an intermediate gray or blue color, while mosaic 

 inheritance yields larger or smaller spots of black and white. 



Mosaic inheritance might also be compared with scalar 

 inheritance. 



Extra^olar Inheritance. — Extrapolar inheritance is a form 

 of descent in which conjugate organisms present characters 

 diverse from those of the parents, instead of being like either 

 parent or intermediate between them. Sueh divergent conju- 

 gates are called heterozygotes in the specialized vocabulary of 

 Mendelism. The fact that such characters depend for their 

 expression, not on the polarity possessed by the gametes before 

 conjugation, but upon the relations assumed during conjugation, 

 enables us to understand the general opinion of Mendelists that 

 heterozygote characters cannot be "fixed" by selection. Simple 



