RULES AND RATIOS OF INHERITANCE 



In earlier chapters we have seen that the dependence of 

 characteristics on genes located in the different chromosomes 

 results in three diverse types of inheritance. Characteristics 

 follow in descent from earlier to later generations the distri- 

 bution of the particular chromosomes in which are the genes 

 on which the characters depend. Since there are three differ- 

 ent types of distribution of the chromosomes, there result 

 three types of inheritance. These are: (i) sex-linked in- 

 heritance, dependent on genes in the X-chromosome; (2) au- 

 tosomal (or typical Mendellan) inheritance, depending on 

 genes in autosomes; (3) Y-chromosome inheritance, depend- 

 ing on genes in Y. 



This dependence of characters on genes in the different 

 chromosomes results, in later generations, in the production 

 of certain numerical ratios among the individuals manifesting 

 different characters. Of these the most common Is the 3 to i 

 ratio occurring in generation F2, in autosomal inheritance. 



In the present chapter we proceed to a systematic examina- 

 tion of the diverse inheritance types and ratios, in their re- 

 lation to the characteristics manifested. 



The most general principle underlying this matter, we 

 shall find, is the following: 



The type and ratio of Inheritance of any character is a 

 relative matter: it depends upon the relative genetic con- 

 stitutions of the two parents and on the characters mani- 

 fested by them. More specifically, the type and ratio shown 



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