CHAPTER XX 

 THE FACTOR HYPOTHESIS AS APPLIED TO ANIMALS 



Immediately after the announcement by De Vries in igoo of the 

 rediscovery of Mendel's paper, zoologists in Europe and in America 

 began experiments in animal breeding with the idea of discovering to 

 what extent Mendel's laws were applicable. It was soon found that 

 the principles of unit characters, dominance, segregation, mono- 

 hybrid, dihybrid, and trihybrid ratios were of practically universal 

 application. A number of instances of Mendelian heredity in animals 

 have already been presented in the preceding chapter and no more 

 simple Mendelian cases need be described. For a considerable period 

 the animal-breeders proceeded no farther in their analysis of the 

 mechanism of heredity than Mendel had done so many years before. 

 In time, however, new facts came to light that needed further analysis, 

 and the older Mendelism was superseded by neo-Mendelism. This 

 new phase in the study of heredity is in the forefront of interest today. 

 Neo-Mendelian heredity in plants has already been discussed. It 

 remains for us to present the data on some phases of neo-Mendelism 

 in animals. 



ILLUSTRATIONS OF THE FACTOR HYPOTHESIS 

 THE FACTORIAL ANALYSIS OF COLOR IN MICE 



Miss Durham, after extensive breeding experiments with numer- 

 ous strains of differently colored mice, has been able to show that 

 the appearance of a particular color in an individual mouse is depend- 

 ent upon the presence or absence of several independently inherited 

 factors, evidently represented by genes in as many different chromo- 

 somes. It seems possible to classify these factors as follows: 



5 = black pigment, which masks chocolate pigment 

 b = absence of B, which gives chocolate 



/ = intensity factor 



i = absence of intensity, or dilution factor 



C = a complementary color factor acting with P 



P = a complementary pigment factor acting with C 



If either C or P is absent, albino mice result no matter what other 

 color factors may be present. 



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