CHAPTER IX 

 THE FACTORIAL HYPOTHESIS 



In Mendelian heredity the word '^factor" is used 

 for something which segregates in the germ cells, 

 and which is somehow connected with particular 

 effects on the organism that contains it. For exam- 

 ple, if a fly (?) with red eyes is crossed to a fly (d^) 

 with white eyes, there will be in F2 three reds to one 

 white, and this ratio can be explained by the assump- 

 tion that in the Fi hybrid something for red eyes 

 has separated from something for white eyes. 



We may express these factorial relations in another 

 way by saying that a germ cell that produces white 

 eyes differs from a germ cell that produces red eyes 

 by one factor-difference. We think of this difference 

 as having arisen through a factor in the red-eyed wild 

 fly mutating to a factor for white. 



Mendelian heredity has taught us that the germ 

 cells must contain many factors that affect the same 

 character. Red eye color in Drosophila, for exam- 

 ple, must be due to a large number of factors, for as 

 many as 25 mutations for eye color at different loci 

 have already come to light. Each produced a specific 

 effect on eye color; it is more than probable that in 

 the wild fly all or many of the normal allelomorphs at 

 these loci have something to do with red eye color. 



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